Don't checkout submodules in bootstrap tests

This doesn't cause any tests to fail, and can greatly speed them up.

src/bootstrap/builder/tests.rs

@@ -7,6 +7,7 @@ fn configure(cmd: &str, host: &[&str], target: &[&str]) -> Config {
     // don't save toolstates
     config.save_toolstates = None;
     config.dry_run = true;
+    config.submodules = Some(false);
     config.ninja_in_file = false;
     // try to avoid spurious failures in dist where we create/delete each others file
     // HACK: rather than pull in `tempdir`, use the one that cargo has conveniently created for us