If defaults are present in gadget.yaml, image.Prepare skips the customizeImage function call

This came up with a customer's testing building custom uc18 images. They were using ubuntu-image with the `--cloud-init` flag, but the cloud-init config was not being copied over into the root filesystem. I looked into the issue and noticed in `image/image_linux.go` in the `setupSeed` function, that `setupSeed` returns before running `customizeImage` when the `defaults` key is used in gadget.yaml. I believe this is a logical error, as that `customizeImage` function should run regardless of the usage of `defaults`.