Construct a plain rustc invocation with no flags set. Note that set_host_compiler_dylib_path
still presets the environment variable HOST_RUSTC_DYLIB_PATH by default.
Construct a plain rustc invocation with no flags set that behaves like a stable compiler,
i.e. rejects unstable flags.
Note that set_host_compiler_dylib_path still presets the environment variable
HOST_RUSTC_DYLIB_PATH by default.