Add mirror selection to "pmbootstrap init"

There is already an open MR in !1810 (closed), which can be used as starting point. I'm creating the issue to give this some more visibility.

postmarketOS has a few binary package mirrors, as listed on https://mirrors.postmarketos.org/ - but we don't have an obvious way to switch mirrors. It would be nice if at least pmbootstrap init offered the choice.

EDIT: the existing, non-obvious way is using the --mirror-pmOS argument with each pmbootstrap call.