Use `x.py check` instead of `cargo check` for build scripts (#1384)
Use `x.py check` instead of `cargo check` for build scripts (#1384)

Cargo check isn't supported and gives an error that CFG_CHANNEL is missing.
It also generates a new target dir and recompiles dependencies.
Use x.py instead, which avoids both issues.
rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide