Key the doc build concurrency group on the pull request number

[albertvillanova]

What does this PR do?

build_pr_documentation.yml keys its concurrency group on github.head_ref, which is the head branch name without the fork owner. Two open PRs from different forks whose branches share a name therefore land in the same group, and a push on one cancels the in-flight doc build on the other.

Shared names are common, because many contributors push to their fork's default branch instead of creating one: 2 of the 36 currently open PRs here already share a head branch name, both on main. A doc build takes around 3 minutes, which is the window in which such a collision cancels a build. I did not find a realized cancellation in the 100 most recent runs, so this is hardening rather than a fix for something currently failing.

github.event.pull_request.number is unique per pull request. This workflow only triggers on pull_request, so the github.run_id fallback is never reached, but keeping it means that a later trigger would fall back to never cancelling rather than to grouping by branch name.

The template this workflow was copied from carries the same expression, and the same one-liner is proposed there in huggingface/doc-builder#829.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[BenjaminBossan]

Thanks!

[hf-security-analysis]

⚠️ Workflow security review — 1 medium, 1 low

Scanned 1 workflow file(s) with zizmor, pinact, OSV/GHSA, Claude, in full, as they stand on the default branch.

.github/workflows/build_pr_documentation.yml

• 🟡 MEDIUM ref-version-mismatch — .github/workflows/build_pr_documentation.yml:14 (via zizmor)
  action's hash pin has mismatched or missing version comment — branch points to commit cf20b09f2743

  Fix: The # vX.Y.Z comment does not match the pinned SHA. Either the comment is stale or the pin was tampered with — re-resolve it.

• 🔵 LOW parse-error — .github/workflows/build_pr_documentation.yml:14 (via pinact)
  pinact could not parse this file: failed to handle a line: SHA-pinned action requires a version comment for verifiability

  Fix: Check the workflow YAML is valid.

Pinning and static findings are deterministic. Findings marked (via Claude) are model judgements — check them before acting.