compile-time regression from switch from num_cpus to available_parallelism needs a regression test

[pnkfelix]

#97549 flagged a regression injected by PR #94524

The problem was fixed by #97911, but we should try to put a test in somewhere that catches if something similar crops up again.

[pnkfelix]

See also conversation from triage meeting https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202022-06-16/near/286362341

[reez12g]

@rustbot claim

[reez12g]

@rustbot release-assignment