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]:
https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/39995
239aa4da8f