Don't compile rustc to self-test compiletest

This was changed from stage 0 to 1 in rust-lang#108905, but I'm not
sure why. Change it to `top_stage` instead to allow people to choose the stage.

This should save quite a bit of time in the `mingw-check` builder, which explicitly runs `x test --stage 0 compiletest`.

Author: jyn514

src/bootstrap/test.rs

@@ -700,7 +700,7 @@ impl Step for CompiletestTest {
     /// Runs `cargo test` for compiletest.
     fn run(self, builder: &Builder<'_>) {
         let host = self.host;
-        let compiler = builder.compiler(1, host);
+        let compiler = builder.compiler(builder.top_stage, host);
 
         // We need `ToolStd` for the locally-built sysroot because
         // compiletest uses unstable features of the `test` crate.