message: Inform build scripts of rustc compiler context
  Fixes
diff: @@ -249,6 +249,13 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
  }
  }
  + // Also inform the build script of the rustc compiler context.
  + cmd.env(
  + "CARGO_RUSTC_WRAPPER",
  + bcx.rustc().wrapper.as_deref().unwrap_or(Path::new("")),
  + );
  + cmd.env("CARGO_RUSTFLAGS", bcx.rustflags_args(unit).join(" "));
  +
  // Gather the set of native dependencies that this package has along with
  // some other variables to close over.
  //
  