unwanted `fish_function_path` full cache invalidation

Version: 4.8.1

For reproducibility and dependency management automation in Nix, I have a program that prefixes the Nix store paths of its dependency Fish programs to the $fish_function_path, and restores the path later.
The program in question is executed 100 times by a helper command that is executed at each shell startup.

This means, the $fish_function_path is modified 200 times on the shell startup each time.

If the cache would not be fully invalidated each time the path is modified, then that would be a noticeable speed boost.

Also, if different $fish_function_path values are also cached, that might even further optimize the shell startup in this case.