Use "rust-installer tarball" to create rustc-src too

This gives us an extra rustc-src.tar.xz, which is 33% smaller than the .tar.gz!

src/bootstrap/dist.rs

@@ -469,13 +469,17 @@ pub fn rust_src(build: &Build) {
  write_file(&plain_dst_src.join("version"), build.rust_version().as_bytes());
 
  // Create plain source tarball
- let tarball = rust_src_location(build);
+ let mut tarball = rust_src_location(build);
+ tarball.set_extension(""); // strip .gz
+ tarball.set_extension(""); // strip .tar
  if let Some(dir) = tarball.parent() {
  t!(fs::create_dir_all(dir));
  }
- let mut cmd = Command::new("tar");
- cmd.arg("-czf").arg(sanitize_sh(&tarball))
- .arg(&plain_name)
+ let mut cmd = rust_installer(build);
+ cmd.arg("tarball")
+ .arg("--input").arg(&plain_name)
+ .arg("--output").arg(sanitize_sh(&tarball))
+ .arg("--work-dir=.")
  .current_dir(tmpdir(build));
  build.run(&mut cmd);