Rollup merge of #83370 - jyn514:setup-tools, r=Mark-Simulacrum

Add `x.py setup tools` which enables `download-rustc` by default

Helps with https://github.com/rust-lang/rust/issues/81930. I know I said in that issue that I should fix that rebasing rebuilds bootstrap, but the compile time improvement is so good I think it's ok to leave that fix for later (I still plan to work on it). I think all the outright bugs have been fixed :)

This builds on https://github.com/rust-lang/rust/pull/83368 so I can set the option to `if-unchanged`.

r? ```@Mark-Simulacrum```