ci: cache frontend build across jobs

.forgejo/workflows/testing.yml

@@ -36,6 +36,11 @@ jobs:
       - run: make checks-frontend
       - run: make test-frontend-coverage
       - run: make frontend
+      - name: "Cache frontend build for playwright testing"
+        uses: actions/cache/save@v4
+        with:
+          path: '/workspace/forgejo/forgejo/public/assets'
+          key: frontend-build-${{ github.sha }}
   test-unit:
     if: ${{ !startsWith(vars.ROLE, 'forgejo-') }}
     runs-on: docker
@@ -84,8 +89,16 @@ jobs:
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
       - uses: ./.forgejo/workflows-composite/setup-env
-      - run: |
-          su forgejo -c 'make deps-frontend frontend deps-backend'
+      - name: "Restore frontend build"
+        uses: actions/cache/restore@v4
+        id: cache-frontend
+        with:
+          path: '/workspace/forgejo/forgejo/public/assets'
+          key: frontend-build-${{ github.sha }}
+      - name: "Build frontend (if not cached)"
+        if: steps.cache-frontend.outputs.cache-hit != 'true'
+        run: |
+          su forgejo -c 'make deps-frontend frontend'
       - uses: ./.forgejo/workflows-composite/build-backend
       - run: |
           su forgejo -c 'make generate test-e2e-sqlite'