"pmbootstrap check" to run same CI scripts locally as done when sending in patches

As part of the considering sourcehut discussions (see the pmOS blog), we figured that it's important to give users the ability to run exactly the same CI scripts locally as it would be done when sending in the patches. (Actually this wouldn't only be useful for SourceHut.)

So the idea is to add a pmbootstrap check command (or similar) that runs just that, and utilizing Alpine chroots as it makes sense.

We'd need it at least for pmbootstrap.git and pmaports.git.