Add Nix.gitignore

[dzmitry-lahoda]

Reasons for making this change:

So nix builds locally some builds (like bin/obj) into result folder.

And also produces links for custom "home" per git repo per user in .devenv.

Links

https://devenv.sh/

https://nixos.org/manual/nix/stable/command-ref/nix-build

[dzmitry-lahoda]

cc @martinwoodward @bdougie @astrojuanlu may you merge this or share with me rights to merge?