Use `make scratch` for building RPMs without tests. This fixes the case,
when the RPM build with test fails to build due to changes in image
manifests. The whole reason of running the script is to regenerate image
test cases when the manifest changed, so this was a chicken and egg
problem.
No CI testing is needed, as this is a development tool.
Signed-off-by: Tomas Hozza <thozza@redhat.com>