functions: nvm.fish: don't attempt to overwrite $fish_user_paths

[jackwestmoretab]

$fish_user_paths is set by the shell at startup, and should not be
overwritten with the -U flag. Doing so results in an error.

Instead, drop the -U flag, and simply append the NVM version bin to the
end of the user's $fish_user_paths.

[jorgebucaran]

@jackwestmoretab Isn't this the same as #107? It isn't. My bad. 😂

[jackwestmoretab]

@jorgebucaran having reviewed #107, I think that the suggestion in #107 (comment) is the right way to go i.e. modify $PATH directly, rather than using $fish_user_paths (for the same reasons given in #107 (comment)).

[jackwestmoretab]

To be clear, I think that the command should manipulate $PATH directly (rather than use set -g ... as the changes in #107 propose).

[jackwestmoretab]

p.s. thanks for linking in #101. I'll close this, as I think proposals there, and in #107, are superior to this!