ci: skip the docs preview for nightly upgrade PRs

Every nightly upgrade PR touches docs/packages/<pkg>.yaml, so all 126 of
them queue a preview deploy behind the single `website` concurrency group.
The surplus runs fail instantly with no jobs at all: tonight 196 of 300 PR
runs failed that way, leaving a red check on PRs whose docs change is one
`- version:` line.

Skip the preview job for those branches. Human PRs keep their preview, and
the deploy stays serialized as before.

Author: luhenry

.github/workflows/website.yml

@@ -26,7 +26,13 @@ jobs:
     steps:
       - run: true
 
+  # The nightly opens one PR per package upgrade, and every one of them
+  # touches docs/packages/<pkg>.yaml. Queueing a preview deploy for each
+  # behind the single `website` lock overflows the concurrency queue and
+  # fails the surplus runs outright, so those PRs get no preview: their doc
+  # change is one `- version:` line.
   preview:
+    if: "!startsWith(github.head_ref, 'github-actions/nightly-upgrade/')"
     needs: [setup]
     runs-on: ubuntu-latest
     permissions: