fix(hygiene action): checkout head ref on forked repos

[aditya-mitra]

git checkout on PR head ref on external repo instead of main repo in the "Lint PR Title and Changesets" github action

example of a run this PR fixes: https://github.com/speakeasy-api/gram/actions/runs/22520577348/job/65326453122?pr=1727

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[vercel]

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 3513415

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[disintegrator]

Will relax this check if a PR comes from a fork but we will not be automatically checking out forked repos in CI.