macs: add apple-virt, nixos-test system features

[cole-h]

[mweinelt]

I think this was tested, and you discussed it in the infra room, but I missed the outcome. Can someone bring me up to speed?

[cole-h]

It was applied to one machine (cosmic-stud) and it "worked" in that they weren't being cancelled / aborted / whatever'd anymore, but the tests taking advantage of this would just hang (c.f. https://github.com/NixOS/nixpkgs/pull/303597#issuecomment-2130121108 and https://github.com/NixOS/nixpkgs/pull/303597#issuecomment-2135345269)

[vcunat]

As it was always hanging, I rolled back the rhea's machines file, so that scheduler wouldn't try to run these jobs.

[Mic92]

Is this good to merge?

[vcunat]

Running those jobs doesn't seem to work. They hang and time out. (I observed it several days ago again.)

[cole-h]

Closing. Feel free to reopen if we want to spend time figuring out why they're hanging.