Use baseos-qe.koji-build.scratch-build.validation as gating.
    
    Replace fedora-ci.koji-build.tier0.functional with
    baseos-qe.koji-build.scratch-build.validation.
    
    The former testcase, fedora-ci.koji-build.tier0.functional, aims to
    perform the kernel rebuild on a fedora CI test system, not in koji.
    
    The latter, baseos-qe.koji-build.scratch-build.validation is supposed
    to run a kernel rebuild in a side-tag koji buildroot, which is closer
    to a real kernel rebuild.
    
    This commit sets baseos-qe.koji-build.scratch-build.validation as
    the default kernel rebuild gating test for binutils.