`srpm`: --offline arg to prevent connecting to Koji
    
    Uses newly added rpkg flag to terminate the action. For some branches,
    offline mode is not possible because there are required macros in Koji.
    
    JIRA: RHELCMP-14513
    Fixes: #600
    
    Signed-off-by: Ondřej Nosek <onosek@redhat.com>