fish: add patch to fix $PATH for nix-darwin

pkgs/shells/fish/default.nix

@@ -148,6 +148,20 @@ let
       hash = "sha256-YUyfVkPNB5nfOROV+mu8NklCe7g5cizjsRTTu8GjslA=";
     };
 
+    patches = [
+      # We don’t want to run `/usr/libexec/path_helper` on nix-darwin,
+      # as it pulls in paths not tracked in the system configuration
+      # and messes up the order of `$PATH`. Upstream are unfortunately
+      # unwilling to accept a change for this and have recommended that
+      # it should be a distro‐specific patch instead.
+      #
+      # See:
+      #
+      # * <https://github.com/LnL7/nix-darwin/issues/122>
+      # * <https://github.com/fish-shell/fish-shell/issues/7142>
+      ./nix-darwin-path.patch
+    ];
+
     # Fix FHS paths in tests
     postPatch = ''
       # src/fish_tests.cpp

pkgs/shells/fish/nix-darwin-path.patch

@@ -0,0 +1,13 @@
+diff --git a/share/config.fish b/share/config.fish
+index d85fd1e185..1d2b5dd598 100644
+--- a/share/config.fish
++++ b/share/config.fish
+@@ -156,7 +156,7 @@
+ # Some things should only be done for login terminals
+ # This used to be in etc/config.fish - keep it here to keep the semantics
+ #
+-if status --is-login
++if status --is-login && not set -q __NIX_DARWIN_SET_ENVIRONMENT_DONE
+     if command -sq /usr/libexec/path_helper
+         # Adapt construct_path from the macOS /usr/libexec/path_helper
+         # executable for fish; see