Add test execution step to x86_64-gnu-aux

Author: reez12g

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

@@ -26,3 +26,5 @@ RUN sh /scripts/sccache.sh
 
 ENV RUST_CONFIGURE_ARGS --build=x86_64-unknown-linux-gnu
 ENV RUST_CHECK_TARGET check-aux
+
+ENV SCRIPT python3 ../x.py test compiler --stage 2