Update branch when automerge failed

[suzuki-shunsuke]

https://github.com/suzuki-shunsuke/renovate-issue-action/actions/runs/3982096380/jobs/6826267482

Run gh -R "$GITHUB_REPOSITORY" pr merge --merge --auto --delete-branch "$PR_NUMBER"
Message: Base branch was modified. Review and try the merge again., Locations: [{Line:1 Column:5[8](https://github.com/suzuki-shunsuke/renovate-issue-action/actions/runs/3982096380/jobs/6826267482#step:3:9)}]
Error: Process completed with exit code 1.

https://github.com/suzuki-shunsuke/reenable-automerge-action/blob/7f64fb47028bfff0f4f1ebf37d2863e59f422336/action.yaml#L16-L24

gh api -X PUT "repos/${GITHUB_REPOSITORY}/pulls/${{ github.event.pull_request.number }}/update-branch"

https://cli.github.com/manual/gh_api
https://docs.github.com/en/rest/reference/pulls#update-a-pull-request-branch