ci: just run stage 2 run-make in `x86_64-gnu-debug

Don't even build a full stage 2, and don't exclude tests with
`--test-args=clang`.

Author: jieyouxu

src/ci/docker/host-x86_64/x86_64-gnu-debug/Dockerfile

@@ -53,5 +53,4 @@ ENV RUST_CONFIGURE_ARGS \
 #     Currently we only run the subset of tests with "clang" in their name.
 
 ENV SCRIPT \
-  python3 ../x.py --stage 2 build && \
-  python3 ../x.py --stage 2 test tests/run-make --test-args clang
+  python3 ../x.py --stage 2 test tests/run-make