Motivation for this change

Fix the broken foreign-env path in the sourceEnv function of fish.nix. Without the fix I get errors like these:

/etc/fish/config.fish (line 6): 
fenv source /etc/fish/foreign-env/shellInit > /dev/null
^
from sourcing file /etc/fish/config.fish
	called on line 39 of file /nix/store/j6k8pvb73n68landa81c173fx45qn4q5-fish-3.1.2/etc/fish/config.fish
from sourcing file /nix/store/j6k8pvb73n68landa81c173fx45qn4q5-fish-3.1.2/etc/fish/config.fish
	called during startup
/etc/fish/config.fish (line 40): 
fenv source /etc/fish/foreign-env/interactiveShellInit > /dev/null
^
from sourcing file /etc/fish/config.fish
	called on line 39 of file /nix/store/j6k8pvb73n68landa81c173fx45qn4q5-fish-3.1.2/etc/fish/config.fish
from sourcing file /nix/store/j6k8pvb73n68landa81c173fx45qn4q5-fish-3.1.2/etc/fish/config.fish
	called during startup

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.