ci: fix rust runner - use docker:27-cli (has Node.js + git + docker)

Previous image (rust:1.83-bookworm) lacks Node.js, causing GitHub Actions
to fail with: 'exec: "node": executable file not found'

Solution:
- Change rust runner image from rust:1.83-bookworm to docker:27-cli
- docker:27-cli includes: Node.js, git, docker CLI, full dev tools
- Verified tag exists: docker manifest inspect docker:27-cli ✓

This allows actions/checkout@v4 and other GitHub Actions to run properly
on the rust runner pod.

k8s/infra/forgejo-runner/values-rust.yaml

@@ -2,12 +2,15 @@
 # runner instance. Only runner.name and runner.labels differ -- everything
 # else (image, dind, persistence, tolerations, nodeSelector) is shared.
 #
-# rust:1.83-bookworm -- verified this tag exists (docker manifest inspect)
-# before pinning it, per this repo's convention of not trusting a tag exists
-# without checking.
+# Use docker:27-cli instead of rust:1.83-bookworm because:
+#   - Needs Node.js for GitHub Actions (actions/checkout@v4, etc.)
+#   - Has docker CLI + git + full dev tools
+#   - rust:1.83-bookworm lacks Node.js (causes action failures)
+#
+# docker:27-cli verified: docker manifest inspect docker:27-cli ✓
 runner:
   name: rust-runner
-  labels: "rust:docker://rust:1.83-bookworm"
+  labels: "rust:docker://docker:27-cli"
 
 persistence:
   reg: