`cargo cbuild` needs to have the `C{C,XX}_FOR_{${platform}}`
variables set just like `cargo` since it is basically a wrapper
around cargo. Without these variables, it will try to use the
`hostPlatform` C compiler to compile `build.rs` scripts, and will
pass flags to that compiler which only make sense on the
`buildPlatform`. So it's just doomed without the environment
variables.
Right now it looks like `rav1e` is the only package we have that is
using `cargo-c`, but if that changes in the future we should factor
this out as its own hook, like `maturinBuildHook` and the others.
4aa036f15a