nvidia-graphics: Add versioned Breaks on initramfs-tools to avoid bug #1076539
initramfs-tools bug #1076539 (duplication of firmware files accessed through directory symlinks) has now been fixed in unstable and in bookworm (in point release 12.7).
This bug was specifically triggered by firmware-nvidia-graphics in conjunction with plymouth or other package that adds graphics drivers to the initramfs.
To ensure that a backport of firmware-nvidia-graphics doesn't trigger this, add a versioned Breaks against unfixed versions of initramfs-tools.