Fish shell integration is broken on older version of fish

[melonedo]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

Commands I ran and their output:

curl -fsSL https://pixi.sh/install.sh | sh

pixi info output:

System
------------
       Pixi version: 0.62.2
        TLS backend: rustls
           Platform: linux-64
   Virtual packages: __unix=0=0
                   : __linux=5.15.0=0
                   : __glibc=2.31=0
                   : __cuda=12.9=0
                   : __archspec=1=skylake
          Cache dir: /home/mlinux/.cache/rattler/cache
       Auth storage: /home/mlinux/.rattler/credentials.json
   Config locations: No config files found

Global
------------
            Bin dir: /home/mlinux/.pixi/bin
    Environment dir: /home/mlinux/.pixi/envs
       Manifest dir: /home/mlinux/.pixi/manifests/pixi-global.toml

Other files (e.g. script files, source files, etc.):

Issue description

Fish shell integration relies on fish_add_path, which is not available on older versions of fish (e.g. the version installed from apt on Ubuntu 20.04). This makes a new fish shell error.

pixi/install/install.sh

Lines 223 to 226 in ff58e6c

	fish)
	LINE="fish_add_path ${PIXI_BIN_DIR}"
	update_shell ~/.config/fish/config.fish "$LINE"
	;;

An alternative would be simply exporting to PATH, or modifying fish_user_paths manually by set -Ua fish_user_paths /home/mlinux/.pixi/bin and then remove ~/.config/fish/config.fish

Expected behavior

Work just like any other shell.