Build proc-macros and run build scripts for the sysroot

[Veykril]

Noticed in #19214, we don't run the build step for the sysroot right now. It wasn't really an issue before as the sysroot didn't rely on non-builtin proc-macros but now it does.

We should consider changing that, though I don't know if we actually can, given this means building the standard library :) Either way its not really necessary right now I believe.