fix: fish shell integration is broken on older version of fish

[tdejager]

Replace fish_add_path with set -gx PATH for compatibility with Fish versions older than 3.2.0 (e.g., Fish 3.1.0 on Ubuntu 20.04).

This is consistent with how bash/zsh PATH setup is handled in the same script.

Description

Fixes #5210

How Has This Been Tested?

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Sonnet

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[tdejager]

@melonedo can you give this a go?

[melonedo]

It should work if I modify ~/.config/fish/config.fish as in the commit, but I am new to pixi so I am not sure how to test the install script itself.

[tdejager]

I think uninstall and reinstalling from the script 🙂

[melonedo]

I think uninstall and reinstalling from the script 🙂

I am sorry, but doesn't install/install.sh download from GitHub releases instead of instead of installing from a local file?

[tdejager]

That does not really matter right? As long as you clone the repository locally and then test the script directly?

[melonedo]

That does not really matter right? As long as you clone the repository locally and then test the script directly?

Yes, AFAICT it works.