gpt-auto-generator: don't wait for udev

Generators run in a context where waiting for udev is not an option,
simply because it's not running there yet. Hence, let's not wait for it
in this case.

This is generally OK to do as we are operating on the root disk only
here, which should have been probed already by the time we come this
far.

An alternative fix might be to remove the udev dependency from image
dissection again in the long run (and thus replace reliance on
/dev/block/x:y somehow with something else).

Fixes: #11205