This new option allows for replacing the sourcing of
/etc/fish/nixos-env-preinit.fish with another file, optionally passing
it through `fenv`. The idea here is that non-NixOS users can do
something like
fish.override {
fishEnvPreInit = sourceBash:
sourceBash "${nix}/etc/profile.d/nix-daemon.sh";
}
and this will set up their shell environment for Nix just as though they
were running NixOS.
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