nixos/mullvad-vpn: change dependency for the daemon to pkg mullvad

[arcuru]

Description of changes

This changes the provider of the mullvad-daemon for the service module from mullvad-vpn to mullvad. The mullvad-vpn package includes the GUI, which is only available on x86-64, while the mullvad package does not and is available cross-platform.

Related: #157207 by @cole-h

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[cole-h]

LGTM :)

[cole-h]

Oh, probably should document this in the release notes. Would you like to do that (edit: in a new PR, of course...), or should I, @patricksjackson?