ci: ensure caches are saved with zstd

work around https://github.com/actions/cache/issues/1169

.forgejo/workflows/testing.yml

@@ -36,6 +36,12 @@ jobs:
       - run: make checks-frontend
       - run: make test-frontend-coverage
       - run: make frontend
+      - name: Install zstd for cache saving
+        # works around https://github.com/actions/cache/issues/1169, because the
+        # consuming job has zstd and doesn't restore the cache otherwise
+        run: |
+          apt-get update -qq
+          apt-get -q install -qq -y zstd
       - name: "Cache frontend build for playwright testing"
         uses: actions/cache/save@v4
         with: