LXD snap not being installed from the right channel when running unminimize

[Impact]
========

There's a step in `unminimize` where it installs the LXD snap. The problem is that it doesn't mention any channel where to install it from. So often it happens that we have the LXD snap on 20.04 or 22.04 from the latest/stable channel (as that's the default!).

However, we should be installing this from stable/ubuntu-20.04 or stable/ubuntu-22.04 (or equivalent) channel.

[Test Plan]
===========

$ lxc launch ubuntu-minimal-daily:jammy jtemp --vm
$ lxc shell jtemp
# apt update
# unminimize
# snap list | grep 'lxd' | awk '{print $4}'

You'll see that it's NOT from stable/ubuntu-$VERSION, whereas it should be. ;)

[Where Problems Could Occur]
=============================

There's very smol room for error especially when the patch only introduces the following to the snap install call: --channel="stable/ubuntu-$VERSION_ID"

But things might behave differently IFF someone has tampered their /etc/os-release OR/AND fiddled with the unminimize script to do a different thing. :)

[OG Bug Report]
===============

In the past, when running unminimize, we'd directly do:
snap install lxd
(cf: https://git.launchpad.net/livecd-rootfs/tree/live-build/auto/build#n322)

which defaults to the latest/stable channel. However, whilst working on enhancing unminimize, it was observed that we install this snap from the stable/ubuntu-<version> channel instead.

This was also noted as a failure when running the CTF tests:
`lxd installed from latest/stable, not stable/ubuntu-23.10`