chore(nix): make home manager module work on flakes

[yusdacra]

[srid]

@yusdacra Was there anything else you had to do? Using your PR on macOS (and Linux) I get:

error (ignored): error: end of string reached
error: the path '~/.gitconfig' can not be resolved in pure mode

Here's my attempt: https://github.com/srid/nixos-config/pull/20

EDIT: looks to be due to use of gitignoreSource: https://github.com/NixOS/nixpkgs/issues/179172

[NorfairKing]

Closing this in favour of #250