Use exit instead of return in fish profile

Older versions of Fish (such as those bundled with Ubuntu LTS 22.04) do
not support return outside of functions. We need to use the equivalent
exit instead.

scripts/nix-profile-daemon.fish.in

@@ -1,6 +1,6 @@
 # Only execute this file once per shell.
 if test -n "$__ETC_PROFILE_NIX_SOURCED"
-  return
+  exit
 end
 
 set __ETC_PROFILE_NIX_SOURCED 1