`rust.debug-assertions = true` should be incompatible with `rust.download-rustc = true | 'if-unchanged'` atm

[jieyouxu]

Before Enable debug assertions on alt builds #131077 lands, the alt rustc builds do not have rustc debug assertions enabled. However, currently rust.download-rustc = true | "if-unchanged" incorrectly assumes that the alt rustc has rustc debug assertions enabled (it does have llvm debug assertions enabled but not rustc debug assertions). This was the reason why #132789 was failing.

See the investigation and #133131 for details.