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 )
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 )

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