Previously the version number was hard coded, with this patch the git
rev is used as the version number. Currently there is no way to get the
git tag for the version number in flakes, so it is not yet possible to
do proper "release" packages with nix flakes.
Also make sure to use the same nixpkgs version for flake-lib as we are
using for building. This is not really necessary, but will decrease the
dependencies to download when building the bngblaster for nix(os).
