Instead of patching the derivation in-place via flags, we just have
a higher order function that takes the Rust package derivation and override it
into a Rustfmt / Clippy oriented derivation: it turns off checks and adds its
required dependencies.
This builds the stub and tool using `rustPlatform.buildRustPackage`
which features a stable Rust compiler, recent enough to support UEFI
targets.
In the future, it will rely on properly defined targets for UEFI in
nixpkgs.