Replace __fish_reconstruct_path hook on first run of __shadowenv_hook to avoid incorrect reordering of PATH

[lilyinstarlight]

PATH entries set via $fish_user_paths get reordered first in $PATH in
fish shell via the __fish_reconstruct_path hook, even if they were added before manually changing $PATH

If we are updating $PATH, we need to replace the __fish_reconstruct_path hook during the first run of __shadowenv_hook (to reduce initial shell startup unexpected behavior for other hooks) such that the __fish_reconstruct_path hook no longer hoists existing $fish_user_paths entries to the front of PATH every change, before we can change $PATH via set -gx PATH

Otherwise the existing contents of the $fish_user_paths variable (set by other hooks) will get erroneously reordered in front of expected shadowenv entries in new shells (including subprocesses started by our tooling in the same shell session)