fkinit: Allow to pass the OTP token via $FKINIT_OTP
    
    This allows me to run:
    
        $ FKINIT_OTP=$(ykman oath accounts code accounts.fedoraproject.org --single) fkinit
    
    I can even alias this:
    
        $ alias fkinit='FKINIT_OTP=$(ykman oath accounts code accounts.fedoraproject.org --single) fkinit'
    
    And the invocation then looks like this:
    
        $ fkinit
        Touch your YubiKey...
        🔐 FAS password: (press TAB for no echo)
    
    Signed-off-by: Miro Hrončok <miro@hroncok.cz>