Protect rhpkg's --arches argument
    
    Invalid arch value fails:
    
        $ rhpkg scratch-build --arches x86_64,noarch
        Could not execute scratch_build: Invalid architecture name:
        x86_64,noarch
    
    Correct works as usual:
    
        $ rhpkg scratch-build --arches x86_64 noarch
    
    Resolves: rhbz#854003