Don't use --all flag with cargo fmt

It adds nothing useful to this project and furthermore,
slows down the test run. Per suggestion in
rust-lang/rustfmt#4247 (comment)

Author: Mikhail Zabaluev

.github/workflows/main.yml

@@ -145,12 +145,6 @@ jobs:
           components: rustfmt, clippy
           override: true
 
-      - name: Restore cargo registry index
-        uses: actions/cache@v1
-        with:
-          path: ~/.cargo/registry/index
-          key: cargo-index-v1-${{ needs.update-deps.outputs.crates-io-index-head }}
-
       - name: Print rustfmt version
         run: rustfmt --version
 
@@ -159,7 +153,13 @@ jobs:
         continue-on-error: false
         with:
           command: fmt
-          args: --all -- --check
+          args: -- --check
+
+      - name: Restore cargo registry index
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/registry/index
+          key: cargo-index-v1-${{ needs.update-deps.outputs.crates-io-index-head }}
 
       - name: Restore dependency crates
         uses: actions/cache@v1