nixos/polkit: Disable pkexec by default

[dasJ]

Motivation for this change

Reduce attack surface.
This is not really tested yet which is why it's a draft.

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/)
• 22.05 Release Notes (or backporting 21.11 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.

[rnhmjoj]

Off the top of my head, this is used by:

• xorg.xf86videointel to control backlight (if xf86-video-intel-backlight-helper is not setuid)
• programs.gamemode
• probably lots of GUI stuff from GNOME and KDE.

[mweinelt]

Then they should be responsible for enabling it. 🤷 And by it I mean polkit in it's entirety, but we're still discussing details over in #156858.