rustbuild: Actually don't build stage0 target rustc
rustbuild: Actually don't build stage0 target rustc

This was attempted in #38853 but erroneously forgot one more case of where the
compiler was compiled. This commit fixes that up and adds a test to ensure this
doesn't sneak back in.

graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust