cli: Print a warning in latest-build if the tag is not a buildroot
    
    If you accidentally use latest-build with a tag argument that is
    not a buildroot, latest-build gives very confusing results.  This
    mistake is easy to make if the tag builds into a buildroot of a
    different name.  With this patch, we now a warning and suggest to
    use the corresponding buildroot tag instead:
    
    $ koji latest-build f32 glibc
    warning: 'f32' is not a buildroot tag
    warning: Did you mean 'f32-build' instead?
    Build                                     Tag                   Built by
    ----------------------------------------  --------------------  ----------------
    glibc-2.30.9000-11.fc32                   f32                   submachine