releng: set default arch for koji-infratag-available
    
    We can't always read the architecture from NEVRA because some of our
    packages are noarch. But we need a specific architecture for
    generating the repo URL, e.g.
    
    https://kojipkgs.fedoraproject.org/repos-dist/f35-infra-stg/latest/x86_64/kojipkgs
    
    With this change, we can easily check all packages for all required
    architectures with just one command.
    
    ./releng/koji-infratag-available --stg \
        python3-copr-common-0.14-1.fc35.noarch.rpm \
        copr-keygen-1.82-1.fc35.noarch.rpm \
        copr-backend-1.153-1.fc35.noarch.rpm \
        copr-dist-git-0.52-1.fc35.noarch.rpm \
        copr-frontend-1.184-1.fc35.noarch.rpm \
        copr-rpmbuild-0.55.2-1.fc35.s390x.rpm \
        copr-rpmbuild-0.55.2-1.fc35.ppc64le.rpm \
        copr-rpmbuild-0.55.2-1.fc35.aarch64.rpm \
        copr-rpmbuild-0.55.2-1.fc35.x86_64.rpm
    
    Merges: #2069