message: don't write a an incorrect rustc version to the fingerprint file
diff: @@ -545,7 +545,11 @@ pub fn prepare_build_cmd<'a, 'cfg>(
  debug!("fingerprint at: {}", loc.display());
  let (local, output_path) = build_script_local_fingerprints(cx, unit)?;
  - let mut fingerprint = Fingerprint { local, ..Fingerprint::new() };
  + let mut fingerprint = Fingerprint {
  + local,
  + rustc: util::hash_u64(&cx.bcx.rustc.verbose_version),
  + ..Fingerprint::new()
  + };
  let compare = compare_old_fingerprint(&loc, &fingerprint);
  log_compare(unit, &compare);
  @@ -648,6 +652,10 @@ fn local_fingerprints_deps(
  }
  fn write_fingerprint(loc: &Path, fingerprint: &Fingerprint) -> CargoResult<()> {
  + debug_assert_ne!(fingerprint.rustc, 0);
  + // fingerprint::new().rustc == 0, make sure it doesn't make it to the file system.
  + // This is mostly so outside tools can reliably find out what rust version this file is for,
  + // as we can use the full hash.
  let hash = fingerprint.hash();
  debug!("write fingerprint: {}", loc.display());
  paths::write(loc, util::to_hex(hash).as_bytes())?;
  