github-ci: use a unique id for the commit check cargo cache

All builds have been using the same cache id for ~/.cargo which
could lead us to conflict situations which is what I think we are
seeing with the commit-check job.

.github/workflows/commits.yml

@@ -14,7 +14,7 @@ jobs:
         uses: actions/cache@v1
         with:
           path: ~/.cargo
-          key: cargo
+          key: commit-check-cargo
       - name: Install dependencies
         run: |
           apt update