fix: add checkout step back to mergeability check

[iCiaran]

Changes

The checkout step was removed in #14454 to force the changed-files action to use the github api to identify changed files, since a local git diff is not reliable from a forked repository.

The checkout is however required before the check so the markdown files can be examined. See recent runs of the workflow, which error in the " Check if any changesets contain minor or major changes" step since the markdown files can't be found for example

Checking for changesets marked as minor or major
cat: .changeset/harden-node-server-defaults.md: No such file or directory

Testing

Docs

Pull request CI step, no docs required.

[changeset-bot]

⚠️ No Changeset found

Latest commit: bdbcd8c

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