Allow skipping the upgrade image verification in UKI mode

[jimmykarily]

In order to check the validity of the upgrade images, we load the whole artifact into memory. With big enough images and low enough memory, this makes the upgrade process die with "OOM killed".

In cases like this, it would be good if we allowed the user to perform this checks out-of-band and make this check optional (skip it with a flag). This would allow people to install in low-memory machines.

[jimmykarily]

We should give a try to the go-uefi peparser to see if it handles this better.