ci: run the github wait job on its own runner label

The github job only polls Gitea for GitHub's commit status, but it holds a
runner slot for as long as the GitHub build takes -- the better part of an
hour for a cold build. On the shared build runners a handful of those
could take every slot and stall real work, so it now runs on the `wait`
label: a runner of its own, with many slots, no docker socket and a small
CPU and memory cap.

.gitea/workflows/ci.yml

@@ -219,7 +219,9 @@ jobs:
   # down): check the mirror, or unset BUILD_ON to build here.
   github:
     if: ${{ vars.BUILD_ON == 'github' }}
-    runs-on: light
+    # Its own runner label with plenty of slots: this job only polls, but holds a slot
+    # for as long as the GitHub build takes, and must not starve the build runners.
+    runs-on: wait
     timeout-minutes: 150
     container:
       image: node:26-bookworm-slim@sha256:582460f614631b59b824ac6020533b9bf339c7fdf3a6d7db31abb6b4065f0212 # 26-bookworm-slim