Disk not being resized for Debian virtual machines

[Linkster78]

Creating a Debian virtual machine with a custom disk size doesn't affect the resulting root partition.

Reproduction steps:

$ incus launch -d root,size=50GiB --vm images:debian/bookworm test
$ incus exec test -- df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            449M     0  449M   0% /dev
tmpfs            94M  612K   93M   1% /run
/dev/sda2       3.8G  939M  2.8G  25% /
tmpfs           467M     0  467M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs            50M   16M   35M  32% /run/incus_agent
/dev/sda1        99M   12M   87M  12% /boot/efi