Adjust `rust.download-rustc` to be not incompatible with `rust.debug-assertions` once alt rustc is built with debug assertions

[jieyouxu]

cc Use download-rustc=false global default, if-unchanged for tools and library profiles, and make rust.debug-assertions=true inhibit downloading CI rustc #133068.

cc Enable debug assertions on alt builds #131077.

[onur-ozkan]

I would just keep it as incompatible as the same problem would occur again when switching this option.

[jieyouxu]

Agreed.