Download lxd image before bootstrap

The first time deploying a bundle many attempts to download the image
will get triggered and the second machine will fail to start and it will
get into a retry strategy, downloading the images beforehand allows to
avoid this and make the CI runs less unstable.

0        pending           pending  bionic      Retrieving image: rootfs: 65% (22.01MB/s)
1        down              pending  bionic      failed to start machine 1 (Failed remote image download: Alias already exists: juju/bionic/amd64), retrying in 10s (10 more attempts)

(cherry picked from commit f1f01dc)

.github/workflows/tox.yaml

@@ -46,6 +46,11 @@ jobs:
         sudo chmod 666 /var/snap/lxd/common/lxd/unix.socket
         # until Juju provides stable IPv6-support we unfortunately need this
         lxc network set lxdbr0 ipv6.address none
+        # pull images
+        lxc image copy --alias juju/bionic/amd64 --copy-aliases ubuntu-daily:bionic local:
+        lxc image copy --alias juju/focal/amd64 --copy-aliases ubuntu-daily:focal local:
+        lxc image copy --alias juju/jammy/amd64 --copy-aliases ubuntu-daily:jammy local:
+        lxc image list
         juju bootstrap --no-gui localhost
     - name: Functional test
       run: |