message: display note to increase debuginfo level when build deps fail
  it's only displayed when backtraces are requested
diff: @@ -405,7 +405,21 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
  },
  true,
  )
  - .with_context(|| format!("failed to run custom build command for `{}`", pkg_descr));
  + .with_context(|| {
  + let mut build_error_context = format!("failed to run custom build command for `{}`", pkg_descr);
  +
  + // If we're opting into backtraces, mention that build dependencies' backtraces can
  + // be improved by setting a higher debuginfo level.
  + if let Ok(show_backtraces) = std::env::var("RUST_BACKTRACE") {
  + if show_backtraces != "0" {
  + build_error_context.push_str("\n\
  + note: To improve backtraces for build dependencies, make sure full debug info is turned on. \
  + More details at https://doc.rust-lang.org/cargo/reference/profiles.html#build-dependencies");
  + }
  + }
  +
  + build_error_context
  + });
  if let Err(error) = output {
  insert_warnings_in_build_outputs(
  