Build manifest tool on mingw-check builder

This builder is not really the correct place to put this, but it
definitely has the time budget and checking this tool builds on just one
platform is more than sufficient.

src/ci/docker/mingw-check/Dockerfile

@@ -20,4 +20,5 @@ COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
 ENV RUN_CHECK_WITH_PARALLEL_QUERIES 1
-ENV SCRIPT python2.7 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu
+ENV SCRIPT python2.7 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu && \
+           python2.7 ../x.py build --stage 0 src/tools/build-manifest