An important point of using C is to write software that adheres to a decades old very widespread standard. Of course developers are free to not do that, but any tiny bit of Rust in the core or even in popular optional code amounts to the same as not using C at all, i.e. only using Rust, as far as portability is concerned.

If your codebase used to conform to a standard and the build environment relies on that standard, and now the your codebase doesn't anymore, then its not the build environment that deviates from the standard, its the codebase that brakes it.