Introduce kernel rebuild CI test.
    
    Introduce baseos-qe.koji-build.scratch-build.validation as a new
    gating test for binutils.  The test makes sure that the binutils
    update under test doesn't break the kernel build process.  The
    testing kernel build happens in koji in a side-tag buildroot
    containing the binutils build under test.
    
    The source code of this new testcase lives here:
    https://github.com/fedora-ci/scratch-build-test
    
    In case of a failure that is considered a false positive, the test
    result can be waived using Bodhi this way:
    https://docs.fedoraproject.org/en-US/ci/gating/#_waive