fix(gha): pass -R to gh pr edit in release_github relabel step

[so0k]

Summary

• The release_github job's "Mark merged release PR as tagged" step calls gh pr edit without an explicit repo, so gh shells out to git to auto-detect it.
• That job runs inside the jsii-terraform container, where the checkout is owned by a different UID than the container user, so git refuses with "detected dubious ownership" and the step fails (exit code 1), even though the GitHub release itself already succeeded.
• Fix: pass -R "${{ github.repository }}" to gh pr edit so it never needs git to infer the repo — smallest possible fix, no container/git config changes needed.

Note: an alternative fix would be adding a git config --global --add safe.directory /__w/cdk-terrain/cdk-terrain step after checkout (as prepare-release and prepare-next already do), but passing -R avoids touching git config in the container entirely.

Diagnosed from the failed run: https://github.com/open-constructs/cdk-terrain/actions/runs/31133236561/job/92730605482

As an immediate unblock, I've also manually relabeled the affected PR (#303) from autorelease: pending to autorelease: tagged so release-please can open the next release PR.

Test plan

• Next release run exercises the release_github job's relabel step and confirms gh pr edit -R ... succeeds inside the container

[github-actions]

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