For run-coverage tests, build rust-demangler with the stage 0 compiler

This avoids useless rebuilds of the demangler when modifying the compiler.

src/bootstrap/src/core/build_steps/test.rs

@@ -1567,9 +1567,11 @@ note: if you're sure you want to do this, please open an issue as to why. In the
         }
 
         if mode == "run-coverage" {
+            // The demangler doesn't need the current compiler, so we can avoid
+            // unnecessary rebuilds by using the bootstrap compiler instead.
             let rust_demangler = builder
                 .ensure(tool::RustDemangler {
-                    compiler,
+                    compiler: compiler.with_stage(0),
                     target: compiler.host,
                     extra_features: Vec::new(),
                 })