Add option to `bootc install to-disk` to not write a random-seed when installing systemd-boot (for bcvk)

[travier]

Current support for systemd-boot uses bootctl to install it into the ESP and this tool will write a random-seed file by default in the ESP. This notably happens when using bcvk to create the image.

When producing disk images for re-use, we should tell bootctl to not write that file using the option: https://www.freedesktop.org/software/systemd/man/latest/bootctl.html#--random-seed=yes%7Cno

Hopefully this will not be a issue most of the time as:

• when using bootc-install for the re-installation/take-over use case, writing the random-seed file is fine and actually a feature.
• when we'll have support in bootupd for shim+systemd then it will not write this file.
• images produced with osbuild/image-builder will likely use bootupd support and thus won't be impacted.

[cgwalters]

We can wire up our existing --generic-image support for this