Force the use of a separte Kerberos cache per thread
    
    When using a single Kerberos cache that is shared among threads,
    Koji logins start failing because the cache gets corrupt. This uses
    the Linux kernel keyring to store a Kerberos cache per MBS thread.
    
    See https://web.mit.edu/kerberos/krb5-1.12/doc/basic/ccache_def.html