"pmbootstrap install" requires git

See #1776 (closed).

The pmbootstrap install uses git to find a revision to use for versioning when it is creating the install package.

It is strange because git is required during initialization of pmbootstrap in the first place.

pmbootstrap init installs git into the native chroot to clone pmaports.git, that is why it works without git being installed on the host system.

So it would be nice if pmbootstrap install worked without having git installed. The easiest solution seems to be to not store pmbootstrap's git hash in /etc/os-release if git is not installed.

(In the long term, we might want to change our approach of the data written to /etc/os-release anyways, right now we put the pmbootstrap version as OS version there, but that should get decoupled. This will be relevant when we have stable releases of postmarketOS.)