ci: Fix .integration_tests_upstream_qemu

ci: Fix .integration_tests_upstream_qemu

We enter $SCRATCH_DIR before going through the process of
cloning QEMU's upstream repo and building it, but once we're
done we don't get back to libvirt's sources, so the very next
step fails with

  /tmp/script.: line 188: ci/jobs.sh: No such file or directory

Use pushd/popd to ensure that we're back to the correct place
once QEMU has been built and installed.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>