Run the git commit message style CI on PRs

[faern]

It was previously set to run on pushes only, and that did not trigger on third party PRs, only on PRs on branches in the original repository

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This change is 

[faern]

Ah, dang. I forgot that this action, if set to run on pull_request also unconditionally checks the PR message here on github. Which is unreasonable to follow the same formatting standards as the git commit messages. There is no sane reason to limit a line to 72 characters in a web markdown field. The automatically inserted reviewable links will also never adhere to this standard.