fix github action, apparently we are running on 20.04 now

this worked yesterday, I promise!

.github/workflows/build.yaml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         image:

scripts/test-in-lxd.sh

@@ -3,10 +3,7 @@ set -eux
 IMAGE=$1
 SCRIPT=$2
 
-snap install lxd --channel=3.0/stable
-
 lxd init --auto
-service lxd restart
 
 lxc launch $IMAGE tester -c security.privileged=true
 lxc config device add tester code disk source=`pwd` path=/subiquity