Need a work around for Nix

[yaitskov]

Hi,

I integrated with git rev. It is working for scenario nix-shell --pure && cabal build, but
for nix-build https://github.com/..../master I get UNKNOWN.

I understand this is not about gitrev itself, because for above scenario doesn't have git.
Tarball is pulled from git repo, but I hope somebody also got this issue and maybe workaround too.