As stated in the official fish tutorial, you can modify the $fish_user_paths universal variable.
Run the following once from the command-line:
set -U fish_user_paths /usr/local/bin $fish_user_paths
This will prepend /usr/local/bin permanently to your path, and will affect the current session and all future instances too because the -U argument will make the variable universal.
From the fish documentation:
… (Note: you should NOT add this line to
config.fish. If you do, the variable will get longer each time you run fish!)fish_user_paths, a list of directories that are prepended to PATH. This can be a universal variable.