Revert compiletest new-executor, to re-land without download-rustc

Revert <https://github.com/rust-lang/rust/pull/139998> because the original
merge triggered download-rustc, which messes with test metrics and prevents us
from properly comparing them before/after the change.

The plan is to re-land this PR as-is, combined with a trivial compiler change
to avoid download-rustc and get proper test metrics for comparison.

This reverts commit be181dd75c83d72fcc95538e235768bc367b76b9, reversing
changes made to 645d0ad2a4f145ae576e442ec5c73c0f8eed829b.