Use direct PATH manipulation to avoid conflicts with user's fish_user_paths

[abap34]

Motivation

The current nix-profile{,-daemon}.fish sets fish_user_paths as a global variable, which shadows the
user's universal fish_user_paths, causing paths added via fish_add_path not to persist across
sessions as many users expect.

Context

This is the same issue that affected asdf:

• bug: fish path set-up breaks fish_add_path asdf-vm/asdf#1629
• fix: use PATH instead of fish_user_paths + tests asdf-vm/asdf#1709

The fix directly manipulates $PATH instead of using fish_user_paths, as noted in
asdf-vm/asdf#1709:

avoids assumptions about $fish_user_paths being global or universal, which should be the shell user's
decision. Note that in fish $PATH cannot be a universal variable (it's simply inherited from the
parent process), so it does not have this issue.

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[abap34]

Hi @edolstra! Thank you for your daily work. Could you review this PR?