Allow retry in get-merged-pr

[qinsoon]

This should fix #1426. I didn't test it, but the retry pattern is the same as in

mmtk-core/.github/workflows/auto-merge-inner.yml

Lines 98 to 103 in 76ef95a

	for n in {1..5}; do
	echo "Wait 30s then try merge"
	sleep 30
	gh pr merge ${{ steps.get-pr.outputs.pr_number }} --auto --repo ${{ inputs.base_repo }} --squash && break
	echo "Failed to merge the PR. Retry..."
	done