Rollup merge of #81520 - jyn514:rustc2, r=Mark-Simulacrum

Don't clone LLVM submodule when download-ci-llvm is set

Previously, `downloading_llvm` would check `self.build` while it was
still an empty string, and think it was always false. This fixes the
check.

This addresses the worst part of https://github.com/rust-lang/rust/issues/76653. There are still some large submodules being downloaded (in particular, `rustc-by-example` is 146 MB, and all the submodules combined are 311 MB), but this is a lot better than the whopping 1.4 GB before.

src/bootstrap/bootstrap.py

@@ -1086,10 +1086,10 @@ def bootstrap(help_triggered):
     else:
         build.set_normal_environment()
 
+    build.build = args.build or build.build_triple()
     build.update_submodules()
 
     # Fetch/build the bootstrap
-    build.build = args.build or build.build_triple()
     build.download_stage0()
     sys.stdout.flush()
     build.ensure_vendored()