[CI] Reach commits outside of upstream repo to calculate coverage

[kvch]

Description

In #996 (fork lghuy05/pgstream): the workflow_run fired and downloaded the artifact, then logged expected exactly one open PR for <sha>, found 0; skipping coverage comment.

From now on we resolve the PR by head repo + branch instead of by commit association:

• Call pulls.list with head: "${wr.head_repository.owner.login}:${wr.head_branch}",
  which works for both fork and same-repo PRs.
• Keep the pr.head.sha === head_sha guard so a superseded push doesn't get
  commented, and the "exactly one match or skip" behavior.
• Drop the now-redundant branch/repo mismatch checks (the head filter already
  scopes the query).

Type of Change

Please select the relevant option(s):

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test coverage improvement
• 🔨 Build/CI changes
• 🧹 Code cleanup

[github-actions]

Coverage

Total: 57.7% (±0.0% vs main)

No package coverage changes in the files this PR touches.