ci: only cargo-clippy a group's latest commit

.github/workflows/clippy.yml

@@ -5,6 +5,13 @@ on:
   pull_request:
   merge_group:
 
+# Ensure that only a single job or workflow using the same concurrency group will run at a time.
+# see https://docs.github.com/en/actions/using-jobs/using-concurrency#example-only-cancel-in-progress-jobs-or-runs-for-the-current-workflow
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  # only needs to check a group's latest commit
+  cancel-in-progress: true
+
 jobs:
   cargo-clippy:
     strategy:
@@ -29,8 +36,9 @@ jobs:
           target/
         key: ${{ matrix.os }}-${{ runner.os }}-${{ runner.arch }}-cargo-clippy-${{ hashFiles('**/Cargo.lock') }}
         restore-keys: |
+          ${{ matrix.os }}-${{ runner.os }}-${{ runner.arch }}-cargo-clippy
           ${{ matrix.os }}-${{ runner.os }}-${{ runner.arch }}-cargo
-    
+
     - name: Install cargo-clippy
       run: rustup component add clippy