Auto merge of #10291 - calebcartwright:cargo-fmt-check, r=ehuss

use new cargo fmt option

As of v1.58, cargo fmt now supports the --check flag directly. Updating it here (and in a few other r-l repos) both because it's more succinct and so more people will see/become aware

.github/workflows/main.yml

@@ -17,12 +17,12 @@ jobs:
     - uses: actions/checkout@v2
     - run: rustup update stable && rustup default stable
     - run: rustup component add rustfmt
-    - run: cargo fmt --all -- --check
+    - run: cargo fmt --all --check
     - run: |
         for manifest in `find crates benches/benchsuite benches/capture -name Cargo.toml`
         do
           echo check fmt for $manifest
-          cargo fmt --all --manifest-path $manifest -- --check
+          cargo fmt --all --manifest-path $manifest --check
         done
 
   test: