message: Fix `--rustc-cfg` not working
  The name of the flag changed for some subcommands from `--cfg` to
  `--rustc-cfg` but the parsing of the arguments was not updated to
  account for the different names.
diff: @@ -473,7 +473,7 @@ pub trait ArgMatchesExt {
  build_config.requested_profile = self.get_profile_name(config, "dev", profile_checking)?;
  build_config.build_plan = self._is_present("build-plan");
  build_config.unit_graph = self._is_present("unit-graph");
  - build_config.cfg = self._is_present("cfg");
  + build_config.cfg = self._is_present("cfg") || self._is_present("rustc-cfg");
  if build_config.build_plan {
  config
  .cli_unstable()
  