do not use render_tests for clippy

Behind the scenes Clippy uses compiletest-rs, which doesn't support the
--json flag we added to Rust's compiletest.

src/bootstrap/test.rs

@@ -767,7 +767,7 @@ impl Step for Clippy {
 
         cargo.add_rustc_lib_path(builder, compiler);
 
-        if add_flags_and_try_run_tests(builder, &mut cargo.into()) {
+        if builder.try_run(&mut cargo.into()) {
             // The tests succeeded; nothing to do.
             return;
         }