Runners crash when building too many rocks

[lucabello]

When a PR is modifying multiple rocks, the rock-pull-request.yaml workflow will try to build all of them. However, without some cleanup we fill up the disk of the runner, which in turn makes CI fail.

We should run at least rockcraft clean (which might be enough if we run it as we build every rock), or something along the lines of:

for instance in $(lxc list --project= rockcraft --format json | jq -r '.[] | select(.name | startswith("rockcraft-")) | .name'); do
    lxc --project=rockcraft delete $instance
done