ci: re-enable cache push step (named-volume bind for relative path)

runner/config.yaml

@@ -18,7 +18,12 @@ cache:
 container:
   network: bridge
   privileged: false
-  options: ""
+  # Bind the binary cache into every job container by referencing the
+  # named volumes defined in compose.yml — those, in turn, are bound
+  # to ./cache/{store,cache.sec} via `${PWD}` so the path is
+  # deployment-relative, not absolute.
+  options: "-v cargoxx-cache-store:/srv/cargoxx-cache/store
+            -v cargoxx-cache-keys:/srv/cargoxx-cache/keys:ro"
   workdir_parent: /workspace
   valid_volumes: []
   docker_host: "unix:///var/run/docker.sock"