perf(ci): skip crossref dump on non-PR builds

The dump only feeds the crossref-tags-bridge artifact, which is itself
only emitted on pull_request_target events (see the existing emit
guard). Skip the dump too — saves a couple seconds per push to master
and bors/staging branches.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: kim-em

.github/workflows/build_template.yml

@@ -748,8 +748,10 @@ jobs:
       - name: dump cross-reference tags to TSV
         # Walks Mathlib.CrossRef.tagExt and writes one TSV row per @[stacks ...] /
         # @[kerodon ...] / @[wikidata ...] tag for the privileged workflow_run
-        # job (crossref_review.yml) to consume. The script has no network access
-        # and enforces a 2 MB output cap.
+        # job (crossref_review.yml) to consume. No network, defence-in-depth
+        # size cap (the trusted bound is in mathlib-ci's post-comment.sh).
+        # Only useful when there's a PR to comment on, so skip on push runs.
+        if: github.event.pull_request.number != ''
         run: lake env lean --run scripts/dump_crossref_tags.lean crossref-tags.tsv
 
       - name: prepare crossref bridge outputs