Allow controlling packager identity through environment variables
    
    In order to support more flexible automation mechanisms, allow
    injecting the packager identity through environment variables as an
    override from the regular detection mechanisms.
    
    Moreover, there are cases where we want to force the fallback identity,
    so we now have a way to force that through an environment variable.
    
    These are intended to be used with multi-user automation systems so
    that the correct identity is set regardless of what the host system
    is actually configured with.
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>