servo: Merge #19883 - Re-enable sccache for Linux builds (from aneeshusa:reenable-sccache-partially); r=jdm

As far as I know, sccache is working properly
on the non-cross-compiling Linux builders.
For safety, only enable it for the builders that run on PRs,
to avoid breaking our nightly generation and scheduled test runs.

This will also allow testing new versions of sccache more easily.
This implements my suggestion from servo/servo#19858 (comment), and should also let us
handle testing a new sccache: rust-lang/rust#42867 (comment) (our current version of sccache [seems to be 2018-01-09](https://github.com/servo/saltfs/blob/f50214b8fa012e03616ecae1ef2913e6fe9044da/servo-build-dependencies/ci-map.jinja#L5)).

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- [ ] These changes fix #__ (github issue number if applicable).

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Source-Repo: https://github.com/servo/servo
Source-Revision: 78ffce1cbe5fcce4d057b69c3cbf0cd2bc2b449c

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 04b98e4452ebe655c59d54f42827b6f3c29b0cd9

Author: aneeshusa

servo/etc/ci/buildbot_steps.yml

@@ -96,40 +96,52 @@ mac-rel-intermittent:
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev:
-  - ./mach clean-nightlies --keep 3 --force
-  - ./mach clean-cargo-cache --keep 3 --force
-  - ./mach test-tidy --no-progress --all
-  - ./mach test-tidy --no-progress --self-test
-  - ./mach build --dev
-  - ./mach test-unit
-  - ./mach package --dev
-  - ./mach build-cef
-  - ./mach build --dev --no-default-features --features default-except-unstable
-  - ./mach build-geckolib
-  - ./mach test-stylo
-  - bash ./etc/ci/lockfile_changed.sh
-  - bash ./etc/ci/manifest_changed.sh
-  - bash ./etc/ci/check_no_panic.sh
+  env:
+    CCACHE: sccache
+    RUSTC_WRAPPER: sccache
+  commands:
+    - ./mach clean-nightlies --keep 3 --force
+    - ./mach clean-cargo-cache --keep 3 --force
+    - ./mach test-tidy --no-progress --all
+    - ./mach test-tidy --no-progress --self-test
+    - ./mach build --dev
+    - ./mach test-unit
+    - ./mach package --dev
+    - ./mach build-cef
+    - ./mach build --dev --no-default-features --features default-except-unstable
+    - ./mach build-geckolib
+    - ./mach test-stylo
+    - bash ./etc/ci/lockfile_changed.sh
+    - bash ./etc/ci/manifest_changed.sh
+    - bash ./etc/ci/check_no_panic.sh
 
 linux-rel-wpt:
-  - ./mach clean-nightlies --keep 3 --force
-  - ./mach clean-cargo-cache --keep 3 --force
-  - ./mach build --release --with-debug-assertions
-  - ./mach test-wpt-failure
-  - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
-  - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
-  - ./mach test-wpt --release --binary-arg=--multiprocess --processes 24 --log-raw test-wpt-mp.log --log-errorsummary wpt-mp-errorsummary.log eventsource
+  env:
+    CCACHE: sccache
+    RUSTC_WRAPPER: sccache
+  commands:
+    - ./mach clean-nightlies --keep 3 --force
+    - ./mach clean-cargo-cache --keep 3 --force
+    - ./mach build --release --with-debug-assertions
+    - ./mach test-wpt-failure
+    - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
+    - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
+    - ./mach test-wpt --release --binary-arg=--multiprocess --processes 24 --log-raw test-wpt-mp.log --log-errorsummary wpt-mp-errorsummary.log eventsource
 
 linux-rel-css:
-  - ./mach clean-nightlies --keep 3 --force
-  - ./mach clean-cargo-cache --keep 3 --force
-  - ./mach build --release --with-debug-assertions
-  - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 2 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
-  - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
-  - ./mach build-geckolib --release
-  - ./mach test-stylo --release
-  - bash ./etc/ci/lockfile_changed.sh
-  - bash ./etc/ci/manifest_changed.sh
+  env:
+    CCACHE: sccache
+    RUSTC_WRAPPER: sccache
+  commands:
+    - ./mach clean-nightlies --keep 3 --force
+    - ./mach clean-cargo-cache --keep 3 --force
+    - ./mach build --release --with-debug-assertions
+    - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 2 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
+    - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
+    - ./mach build-geckolib --release
+    - ./mach test-stylo --release
+    - bash ./etc/ci/lockfile_changed.sh
+    - bash ./etc/ci/manifest_changed.sh
 
 linux-nightly:
   - ./mach clean-nightlies --keep 3 --force