workflows/ci: use ubuntu 20.04 for cgroup v1 functional test

workflows/ci: use ubuntu 20.04 for cgroup v1 functional test

Using ubuntu-latest (Ubuntu 22.04) for the cgroup v1 functional test
fails, due to two reasons:
- The lxd package has been renamed to lxd-installer
- Ubuntu-latest (22.04) supports only cgroup v2 hierarchy and for
  branch-2.0 pass the test cases, requires forward porting a couple of
  patches from 3.0 branch. Let's run it on the known stable release
  Ubuntu-20.04 instead.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>