fish: source NixOS environment on non-login shells, when it hasn't been

sourced
(this fixes issue #25789:
https://github.com/NixOS/nixpkgs/issues/25789#issuecomment-301577290 and
the issue with git-annex mentioned here
https://github.com/NixOS/nixpkgs/pull/24314#issuecomment-301587124 )