Sign commits

[gdrouet]

Subject of the issue

It seems we cannot sign commits.

Steps to reproduce

Simply create a PR with the action, added commit won't be signed.

[peter-evans]

Hi @gdrouet

Please see this section of the docs:
https://github.com/peter-evans/create-pull-request/blob/main/docs/concepts-guidelines.md#gpg-commit-signature-verification