Use Builder::tool_exe to build the coverage-dump tool

This appears to be the canonical way to build a tool with the stage 0 compiler.

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

@@ -1782,10 +1782,7 @@ fn run(self, builder: &Builder<'_>) {
         }
 
         if matches!(mode, "coverage-map" | "coverage-run") {
-            let coverage_dump = builder.ensure(tool::CoverageDump {
-                compiler: compiler.with_stage(0),
-                target: compiler.host,
-            });
+            let coverage_dump = builder.tool_exe(Tool::CoverageDump);
             cmd.arg("--coverage-dump-path").arg(coverage_dump);
         }