Fix tests.evalNixOS
Fix tests.evalNixOS

This failed because NixOS' release.nix calls builtins.fetchGit.

vcunat/nix,NixOS/nix,peti/nix,NixOS/nix,rycee/nix,zimbatm/nix,layus/nix,zimbatm/nix,peti/nix,dtzWill/nix,ehmry/nix,NixOS/nix,peti/nix,layus/nix,dezgeg/nix,ehmry/nix,peti/nix,ehmry/nix,shlevy/nix,dtzWill/nix,rycee/nix,rycee/nix,shlevy/nix,shlevy/nix,pikajude/nix,dtzWill/nix,ehmry/nix,pikajude/nix,zimbatm/nix,dtzWill/nix,rycee/nix,pikajude/nix,vcunat/nix,layus/nix,zimbatm/nix,dezgeg/nix,shlevy/nix,shlevy/nix,dtzWill/nix,ehmry/nix,dezgeg/nix,ehmry/nix,layus/nix,vcunat/nix,layus/nix,vcunat/nix,rycee/nix,peti/nix,NixOS/nix,dezgeg/nix,dezgeg/nix,NixOS/nix