lowfi: mpris feature only on Linux

[poulter7]

Only include the mpris build feature on Linux.

This makes the build consistent with lowfi's stated support: https://github.com/talwat/lowfi?tab=readme-ov-file

"MPRIS is currently optional feature in cargo (enabled with --features mpris) due to it being only for Linux"
and indeed this will build but not run on Darwin without this change.

With this change, the binary works on Darwin.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • N/A [ ] (Package updates) Added a release notes entry if the change is major or breaking
  • N/A [ ] (Module updates) Added a release notes entry if the change is significant
  • N/A [ ] (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[ZarredFelicite]

Thanks!