fix: install LXD snap from stable/ubuntu-<version> channel (LP: #2036725)

In the past, we'd directly snap install lxd 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`

(cherry picked from commit 12a2109c22)

live-build/auto/build

@@ -288,8 +288,9 @@ fi
 echo "Removing lxd installer package..."
 apt-get purge -y lxd-installer
 
-echo "Installing lxd from snap..."
+. /etc/os-release
-snap install lxd
+echo "Installing lxd from snap from stable/ubuntu-$VERSION_ID channel"
+snap install --channel="stable/ubuntu-$VERSION_ID" lxd
 EOF
 		fi
 		cat >> chroot/usr/local/sbin/unminimize <<'EOF'