Default experimental_toolchain_generated_sysroot to True (#2277)

#2223 only sets
`--@rules_rust//rust/settings:experimental_toolchain_generated_sysroot`
in a few targets on CI. This PR defaults the feature flag to `True`.

This ensures the feature flag works at a larger scope because we remove
it.

---------

Co-authored-by: UebelAndre <[email protected]>

Author: Vinh Tran

.bazelci/presubmit.yml

@@ -92,10 +92,8 @@ tasks:
     test_targets: *default_linux_targets
     build_flags:
       - "--compilation_mode=opt"
-      - "--@rules_rust//rust/settings:experimental_toolchain_generated_sysroot=True"
     test_flags:
       - "--compilation_mode=opt"
-      - "--@rules_rust//rust/settings:experimental_toolchain_generated_sysroot=True"
   macos_opt:
     name: Opt Mode
     platform: macos

rust/settings/BUILD.bazel

@@ -68,6 +68,6 @@ bzl_library(
 # A flag to set rustc --sysroot flag to the sysroot generated by rust_toolchain
 incompatible_flag(
     name = "experimental_toolchain_generated_sysroot",
-    build_setting_default = False,
+    build_setting_default = True,
     issue = "https://github.com/bazelbuild/rules_rust/issues/2039",
 )