Add the Flake Parts tree into the testbed environment, enabling
realistic testing interactions.
Flake Parts is chosen over Stylix to reduce cache invalidations, and
over other flake inputs as the smallest tree.
Allow choosing the testbed desktop, ideally as a temporary solution
until migrating to the cage environment.
Link: https://github.com/nix-community/stylix/pull/1222
Reviewed-by: Flameopathic <64027365+Flameopathic@users.noreply.github.com>
Reviewed-by: awwpotato <awwpotato@voidq.com>
Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>
