CI: Limit job concurrency

We are seeing random CI failures with:

    Failed to set execute bit on remote files

Limit concurrency in case it is related to concurrent access to the same
Docker image.

Author: priteau

.github/workflows/ci.yml

@@ -38,6 +38,8 @@ jobs:
     name: Molecule
     runs-on: ubuntu-22.04
     needs: build
+    concurrency:
+      group: matrix-job-${{ github.ref }}-${{ matrix.image }}
     strategy:
       fail-fast: false
       matrix: