bootstrap: relax compiler-rt root assertion

Not needed during tests.

Author: jieyouxu

src/bootstrap/src/core/build_steps/compile.rs

@@ -590,7 +590,12 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
                 ),
             );
             let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
-            assert!(compiler_builtins_root.exists());
+            if !builder.config.dry_run() {
+                // This assertion would otherwise trigger during tests if `llvm-project` is not
+                // checked out.
+                assert!(compiler_builtins_root.exists());
+            }
+
             // The path to `compiler-rt` is also used by `profiler_builtins` (above),
             // so if you're changing something here please also change that as appropriate.
             cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);