Make envkernel.sh usable outside source tree

This makes it possible to use envkernel.sh without sourcing it from a full source tree as long as pmbootstrap is available in PATH (e.g. from a distro package). In this case the pmbroot and pmbootstrap aliases won't be set, and neither the pmbootstrap_dir environment variable.

Additionally install envkernel.sh as a data file, so people will get it when installing via pip or (likely) distro packages.

Closes: #1909 (closed)