Allow overriding the version via env var and/or file

[lefou]

Sometimes, it's wanted to override the version which is normally inferred from the version control system (git). E.g. when building from a release tarball.

Therefore, mill-vcs-version should check for a MILL_VCS_VERSION env variable as well as for a MILL_VCS_VERSION file. If any of those is present, it should read the version from these instead of trying to run git.