bumblebee service: Fix pmMethod bbswitch check

[zraexy]

Motivation for this change

Made useBbswitch work when pmMethod is "auto" and removed invalid pmMethod option "nouveau".

Without this when using the nvidia drivers bumblebeed looks for bbswitch and fails to find it because it is not included.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@zraexy, thanks for your PR! By analyzing the history of the files in this pull request, we identified @paraseba, @abbradar and @ktosiek to be potential reviewers.

[abbradar]

Ouch, nouveau is accidential leak of my out-of-tree patches. Thanks!