message: Include toolchain specification in error message
diff: @@ -200,11 +200,19 @@ fn detect_sysroot_src_path(target_data: &RustcTargetData<'_>) -> CargoResult<Pat
  .join("rust");
  let lock = src_path.join("Cargo.lock");
  if !lock.exists() {
  - anyhow::bail!(
  + let msg = format!(
  "{:?} does not exist, unable to build with the standard \
  library, try:\n rustup component add rust-src",
  lock
  );
  + match env::var("RUSTUP_TOOLCHAIN") {
  + Ok(rustup_toolchain) => {
  + anyhow::bail!("{} --toolchain {}", msg, rustup_toolchain);
  + }
  + Err(_) => {
  + anyhow::bail!(msg);
  + }
  + }
  }
  Ok(src_path)
  }
  