[CI] check-links fails due to unrelated changes

[djaglowski]

The check-links job often fails on pull requests due to unrelated changes because it is comparing the branch to origin/main, rather than to a common ancestor with origin/main. This means that changes made on recently merged PRs will be picked up as part of the diff, and therefore validation will run against those files as well. (example)

I believe the solution is to use git merge-base to identify the appropriate common ancestor between origin/main and HEAD.