* changes: Revert^3 "add rust_toolchain_rustc_prebuilt module type" Revert^3 "add crate_root property to rust modules" Revert^3 "allow Ninja variables in RuleBuilder API" Revert "conditionally escape rule builder command" Revert "support sandboxed rust rules" Revert "fix failing rust_aconfig_library test" Revert "rustSetToolchainSource to use linux-x86 srcs" Revert "remove rust deps on clang prebuilts"