enable parallel rustc front end in nightly builds Refers to the [MCP](https://github.com/rust-lang/compiler-team/issues/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`