Auto merge of #117435 - SparrowLii:nightly_parallel, r=oli-obk,davidtwco

enable parallel rustc front end in nightly builds

Refers to the [MCP](rust-lang/compiler-team#681), this pr does:
1. Enable the parallel front end in nightly builds, and keep the default number of threads as 1. Then users can use the parallel rustc front end via -Z threads=n option.

2. Set it up to serial front end for beta/stable builds via bootstrap.

3. Switch over the alt builders from parallel rustc to serial, so we have artifacts without parallel to test against the artifacts with parallel.

r? `@oli-obk`

cc `@cjgillot` `@nnethercote` `@bjorn3` `@Kobzol`

Author: bors