bug: Fish module duplicates & reorders XDG_DATA_DIRS on SteamOS

[Labaman]

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• My Nixpkgs and Home Manager versions are in sync

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Issue description

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programs.fish behaves like programs.bash: on activation it reorders $XDG_DATA_DIRS and adds Nix paths at both the start and end, creating duplicates and breaking expected overrides on SteamOS (see #8076). Unlike the bash module, fish affects only $XDG_DATA_DIRS (not $PATH). On SteamOS, this causes the “Install Firefox” desktop entry to take precedence again after Flatpak Firefox is installed. Shell modules shouldn’t change XDG path order— especially with targets.genericLinux.enable = true — nor introduce duplicates when variables are already set by other Nix/Home Manager sources.

Maintainer CC

@rycee

System information

this path will be fetched (0.00 MiB download, 0.00 MiB unpacked):
  /nix/store/8qvvplwh5hmywq4f1n6cs21r32599jnp-nix-info
copying path '/nix/store/8qvvplwh5hmywq4f1n6cs21r32599jnp-nix-info' from 'https://cache.nixos.org'...
 - system: `"x86_64-linux"`
 - host os: `Linux 6.11.11-valve24-2-neptune-611-gfd0dd251480d, SteamOS, noversion, 20250903.1`
 - multi-user?: `no`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.32.2`
 - nixpkgs: `/nix/store/5xik13nazk3xh67mgrrgr4czw9m1a9rx-nixpkgs/nixpkgs`