Restore Matrix concurrency check

Author: backspace

.github/workflows/ci.yaml

@@ -180,10 +180,9 @@ jobs:
   matrix-client-test-2:
     name: Matrix Client Tests - Group 2
     runs-on: ubuntu-latest
-    # FIXME restore
-    # concurrency:
-    #   group: matrix-client-test-2-${{ github.head_ref || github.run_id }}
-    #   cancel-in-progress: true
+    concurrency:
+      group: matrix-client-test-2-${{ github.head_ref || github.run_id }}
+      cancel-in-progress: true
     steps:
       - uses: actions/checkout@v3
       - uses: ./.github/actions/init