message: Reinstate CARGO_PRIMARY_PACKAGE
diff: @@ -530,6 +530,11 @@ fn prepare_rustc(
  let mut base = cx
  .compilation
  .rustc_process(unit, is_primary, is_workspace)?;
  +
  + if is_primary {
  + base.env("CARGO_PRIMARY_PACKAGE", "1");
  + }
  +
  if cx.bcx.config.cli_unstable().jobserver_per_rustc {
  let client = cx.new_jobserver()?;
  base.inherit_jobserver(&client);
  