Document build script envvars for rustc context
Document build script envvars for rustc context

rust-lang/cargo,eminence/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo