Queue tracker PR builds instead of cancelling them (#902)

The tracker opens its PR and then labels it, so a new tracker PR starts
two build runs at once. The second cancelled the first, whose always-run
`result` job then failed and left a red required check on the PR (#901)
until the surviving run finished. The tracker also force-pushes every two
hours, which would cancel long builds the same way the sync branches did.

Treat auto/track-branches like auto/sync-*: the newer run waits and
reuses the finished build's artifacts.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/upstream-sources.md

@@ -229,9 +229,9 @@ These packages were already excluded from automatic AUR updates. The migration p
 `sync-upstream.yml` pushes its PR with the built-in `GITHUB_TOKEN`, so GitHub
 holds the PR's build and test runs for approval. The workflow labels its own
 PR `build-approved`, and its `approve` job releases the held runs for each
-commit it pushes. A push to an `auto/sync-*` branch does not cancel the PR's
-build in flight: the new build waits, then reuses every artifact the finished
-one uploaded.
+commit it pushes. A push to an `auto/sync-*` branch, or to the tracker's
+`auto/track-branches`, does not cancel the PR's build in flight: the new
+build waits, then reuses every artifact the finished one uploaded.
 
 Because of that push, GitHub starts no `pull_request_target` workflow for the
 PR, so `auto-merge-pr.yml` never arms it: a maintainer merges it.