ci: merge e2e workflow into testing.yml

ci: cache frontend build across jobs

ci: ensure caches are saved with zstd

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

ci: require unit tests for remote cacher

- prevents unnecessary runs in case the unit tests already fail
- starts the integration tests about 2 minutes earlier
- should give some overall speedup to the CI run, because the long integration tests are run and finish earlier, and the cacher tests should still usually finish in time
- does not save any computing resources, just provides quicker results when runners are not under high load

.forgejo/workflows-composite/build-backend/action.yaml

@@ -6,7 +6,7 @@ runs:
     - uses: actions/cache@v4
       id: cache-backend
       with:
-        path: '/workspace/forgejo/forgejo/gitea'
+        path: ${{github.workspace}}/gitea
         key: backend-build-${{ github.sha }}
     - if: steps.cache-backend.outputs.cache-hit != 'true'
       run: |

.forgejo/workflows-composite/setup-env/action.yaml

@@ -4,7 +4,8 @@ runs:
     - name: setup user and permissions
       run: |
         git config --add safe.directory '*'
-        adduser --quiet --comment forgejo --disabled-password forgejo
+        # ignore if the user already exists (like with the playwright image)
+        adduser --quiet --comment forgejo --disabled-password forgejo || true
         chown -R forgejo:forgejo .
     - uses: https://codeberg.org/fnetx/setup-cache-go@b2214eaf6fb44c7e8512c0f462a2c3ec31f86a73
       with: