snapcraft --use-lxd is broken

[Omer Akram]

It was working a day before yesterday but since yesterday its not working. It also broke our CI.

Snapcraft seems to have hard-coded path to the lxd image it seems. The specific image doesn't exist anymore hence the issue.

```
om26er@X1C6:~/scm/crossbario/crossbarfx$ snapcraft --use-lxd
Launching a container.
Build environment is in unknown state, cleaning first.
An error occurred with the instance when trying to launch with 'LXD': Unable to fetch https://cloud-images.ubuntu.com/buildd/daily/focal/20200728/focal-server-cloudimg-amd64-lxd_combined.tar.gz: 404 Not Found.
Ensure that 'LXD' is setup correctly and try again.
```

[Omer Akram (om26er)]

The image clearly doesn't exist here https://cloud-images.ubuntu.com/buildd/daily/focal/

[Sergio Schvezov (sergiusens)]

Sorry you saw this, we fixed it on the 10th and it was related to a publication issue that showed up tied to the 20.04.1 release. This issue should not happen again.