aportgen: fix fork_alpine related error
Make sure, that "args.fork_alpine" is always present. Otherwise, pmbootstrap will fail if the aportgen code is called by anything but "pmbootstrap aportgen". For example, when the user is adding a new device during "pmbootstrap init".
Fixes: 54e51759 ("aportgen: add feature to fork upstream packages")
Thanks to xred_cubeX for reporting this bug in #postmarketOS.