ci: check src/tools/bump-stage0 in pr-check-2

To catch obvious build failures of the `src/tools/bump-stage0` early,
before we find out it can't even build when we really need it to work.

Author: jieyouxu

src/ci/docker/host-x86_64/pr-check-2/Dockerfile

@@ -28,6 +28,7 @@ RUN sh /scripts/sccache.sh
 
 ENV SCRIPT \
         python3 ../x.py check && \
+        python3 ../x.py check src/tools/bump-stage0 && \
         python3 ../x.py clippy ci --stage 2 && \
         python3 ../x.py test --stage 1 core alloc std test proc_macro && \
         python3 ../x.py test --stage 1 src/tools/compiletest && \