ci: re-enable actions cache now that the runner serves it

The Gitea Actions cache service now answers on 10.200.0.1:43513
(post nftables fix on the runner side). Turn `cache: true` and
`cache: pnpm` back on so setup-go/setup-node can use it for
cross-job tarball caching on top of the host-persistent caches we
already rely on.

The setup-* actions still tolerate the cache being unavailable, so
this is reversible to `cache: false` if the service goes away again.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/prod-build.yaml

@@ -37,8 +37,7 @@ jobs:
         uses: actions/setup-go@v5
         with:
           go-version-file: go.work
-          # See go-unit.yaml for why `cache: true` is disabled.
-          cache: false
+          cache: true
 
       - name: Set up pnpm
         uses: pnpm/action-setup@v4
@@ -49,7 +48,8 @@ jobs:
         uses: actions/setup-node@v4
         with:
           node-version: 22
-          # See dev-deploy.yaml for why `cache: pnpm` is disabled.
+          cache: pnpm
+          cache-dependency-path: ui/pnpm-lock.yaml
 
       - name: Resolve image tag
         id: tag