robosignatory: add f39 signing before branching
    
    This is an attempt to allow us to sign f38 with the f39 key also in
    addition to the f38 one. Using this we can make a ugly loop that signs
    all the stuff in the f38 tag with the f39 key. (Hopefully)
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>