CI: Support not having "pmbootstrap" in $PATH

Not everyone's setup includes "pmbootstrap" as a command in $PATH. Currently, in such setups, $ pmbootstrap ci kconfig (and potentially other testcases) in the pmaports repository do not work as they expect the "pmbootstrap" command to exist. Resolve this by checking whether PMBOOTSTRAP_CMD is set and use that as command for invoking pmbootstrap. If it isn't set (because you're using an old pmbootstrap version or invoking a script directly for some reason), fall back to the old behaviour as to not break compatibility.

Depends on pmbootstrap patch that sets PMBOOTSTRAP_CMD to argv[0].

pmbootstrap patch: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/39995