do not use run.path(compiler/....) for RustOffload::should_run not to build it unexpectedly

Author: sgasho

src/bootstrap/src/core/build_steps/llvm.rs

@@ -966,7 +966,7 @@ impl CommandLineStep for RustOffload {
     const IS_HOST: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.path("compiler/rustc_llvm/llvm-wrapper/offload")
+        run.alias("rust-offload")
     }
 
     fn make_run(run: RunConfig<'_>) {