[Bug 2036725] Re: LXD snap not being installed from the right channel when running unminimize

[Utkarsh Gupta]

It turned out that since `lxd-installer` will need to be modified to
pull the snap from the right channel when we know it (whatever channel
is used when the LTS is released), it might be best to rely on `lxd-
installer` to do the right thing.

For LTS, the snap channel doesn't necessarily would be XX.04. For instance,
For jammy, it is: 5.0/stable/ubuntu-22.04 (set in the seed definition)
For focal, it is: 4.0/stable/ubuntu-20.04 (set in the seed definition)
...and so on.

OTOH, for non-LTS releases, there's no set channel. For eg: see Groovy,
Impish, et al, et al.

As such, we propose to simply run the `lxd-installer` prior to purging
it as that will do the right thing from the right channel.

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https://bugs.launchpad.net/bugs/2036725

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

Status in livecd-rootfs package in Ubuntu:
  New
Status in livecd-rootfs source package in Jammy:
  New
Status in livecd-rootfs source package in Lunar:
  Fix Committed

Bug description:
  [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`

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