Rollup merge of #130306 - onur-ozkan:avoid-submodule-updates-in-tests…

…, r=Kobzol

avoid updating LLVM submodule during bootstrap unit tests

To test this, make sure you don't have `src/llvm-project` fetched and then set `llvm.download-ci-llvm=true` and run `x test bootstrap`.

src/bootstrap/src/core/config/config.rs

@@ -2739,6 +2739,8 @@ impl Config {
                 return false;
             }
 
+            // Fetching the LLVM submodule is unnecessary for self-tests.
+            #[cfg(not(feature = "bootstrap-self-test"))]
             self.update_submodule("src/llvm-project");
 
             // Check for untracked changes in `src/llvm-project`.