rust: shipped mingw-w64-gcc is too old to link i686-pc-windows-gnu objects

(related: #381)

The version shipped with Cerbero is GCC 8.3.0, which is too old to link the object files produced by Rust.

As of this writing, the rust-mingw component supplies such a linker, which is GCC 13.2. However, installing this component requires the toolchain to be switched to stable-i386-pc-windows-gnu. (This is a rustup bug for which I'll be filing an issue upstream after this.)