fish: pick up completion files from other packages

Some packages bring their own completions in
/share/fish/vendor_completions.d. Now they are picked up by fish from
every path in NIX_PROFILES.

nixos/modules/programs/fish.nix

@@ -101,6 +101,9 @@ in
       end
     '';
 
+    # include programs that bring their own completions
+    environment.pathsToLink = [ "/share/fish/vendor_completions.d" ];
+
     environment.systemPackages = [ pkgs.fish ];
 
     environment.shells = [