Add filesystem configuration matching Proxmox image builder output
to allow template2 to build with both `nixos-rebuild build` and
`nixos-rebuild build-image --image-variant proxmox`.
Filesystem specs discovered from running VM:
- ext4 filesystem with label "nixos"
- x-systemd.growfs option for automatic partition growth
- No swap partition
Using lib.mkDefault ensures these definitions work for normal builds
while allowing the Proxmox image builder to override when needed.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Torjus Håkestad
2aeed8f231
