Bugfix in PR workflow job filtering

[anjbur]

Description

Bugfix for PR workflow filtering. Currently, it compares the PR branch against base.sha, which resolves as the most recent commit on the PR's base branch (i.e. main). If the PR branch hasn't pulled the most up-to-date version of main, then this diff will include any changes made on main since they last pulled an update. Changing this to base.ref instead resolves to the base branch name (main) and the diff will be taken against the merge-base.

In short, this will prevent unnecessary jobs running in the CI when a feature branch is pushed without the most recent changes on main.

See the triggered CI checks below on 00c520e for confirmation of the bug.

Runtime / performance impact

Self-review checklist

Please confirm each item before requesting review. Check [x] or strike
through and explain.

Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
• CI logs reviewed; no unexplained warnings or errors.
• Full CI has been run.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing.
• Negative tests added where exceptions are expected.
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness.
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs.
• User-visible behavior changes have public docs, or a follow-up is
  tracked.
• User-facing docs for new features are in a separate PR held until
  release (the docs site publishes immediately on merge to the default
  branch, so feature docs must not land before the feature ships).

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.