Add lxc image list to integration tests (#47)

Make it easier to track changes to LXC images downloaded by Juju to help
troubleshoot issues such as
https://chat.canonical.com/canonical/pl/spbm6i4hwprrbqb13e9tfuuuyc

Author: carlcsaposs-canonical

.github/workflows/integration_test_charm.yaml

@@ -144,6 +144,11 @@ jobs:
       - timeout-minutes: 1
         if: ${{ success() || (failure() && steps.spread.outcome == 'failure') }}
         run: snap list
+      - name: lxc image list
+        timeout-minutes: 1
+        if: ${{ (success() || (failure() && steps.spread.outcome == 'failure')) && inputs.path-to-charm-directory == 'machines' }}
+        # sudo needed since spread runs scripts as root
+        run: sudo lxc image list
       - name: Select model
         timeout-minutes: 1
         # `!contains(matrix.job.spread_job, 'juju29')` workaround for juju 2 error: