Workflow failure due to Ubuntu 20 image retirement

I see the workflow is failing due to github having [retired](https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/) their ubuntu 20 images. I also see there's a problem with simply updating to the more recent ubuntu 22 images, since they're running kernel 6.8 and as a consequence the syscall path for creating a file descriptor would no longer be tested.

I can see at least two options for addressing this problem:

1.) Modify the public API of the crate to allow the user to choose which method to use - either use the syscall or the ioctl.
2.) Introduce features, much like the one introduced in this PR, controlling which methods of creating a file descriptor are allowed.

_Originally posted by @ureeves in https://github.com/bytecodealliance/userfaultfd-rs/pull/70#issuecomment-2888697139_