Mark pre-releases via -git (or similar)

[wisp3rwind]

As noted in #4395, it might be useful to the following, so it's always obvious when someone is running a version installed from git:

We have not previously had the workflow set up to append -git or similar to pre-release versions. Setting that up in our release.py automation could be a useful addition.

Originally posted by @sampsyo in https://github.com/beetbox/beets/issues/4395#issuecomment-1172913377