Baseline-version release workflow opens a new PR per release instead of updating the existing open one

## Summary

The release workflow that bumps `PackageValidationBaselineVersion` opens a brand-new PR for every release instead of updating an already-open baseline PR. This leaves a trail of stale, superseded baseline PRs that have to be closed by hand.

## Repro

1. Run the release workflow (`.github/workflows/release-github-tasks.yml`, `baseline-pr` job) for consecutive patch releases.
2. Observe the PRs it creates:
- #17882 — Update PackageValidationBaselineVersion to 13.4.1 (closed)
- #17887 — Update PackageValidationBaselineVersion to 13.4.2 (closed)
- #18026 — Update PackageValidationBaselineVersion to 13.4.3 (open)

## Expected vs. actual

**Expected:** If a baseline-update PR is already open, the workflow updates that PR (new version bump pushed onto it) rather than opening another one. At most one open baseline PR at a time.

**Actual:** A new PR is opened for each release. The "Check for Existing Baseline PR" step keys off the version-specific branch name `update-baseline-${release_version}`, so it only matches a PR for the *same* version and never finds the previous release's still-open PR. Superseded baseline PRs accumulate and must be closed manually.

cc @joperezr