[DPE-5480] Add base in test_multi_relations to workaround libjuju bug (…

…#506)

* [DPE-5480] Add base to test_multi_relations.py to workaround libjuju issue

The test is randomly failing on libjuju 3.5.2.0 with Juju 3.6-beta2,
it is a known issue reported as juju/python-libjuju#1090

As a (temporary?) workaround lets add a base here.

tests/integration/test_multi_relations.py

@@ -42,13 +42,15 @@ async def test_build_and_deploy(ops_test: OpsTest):
             num_units=1,
             channel="latest/edge",
             config=config,
+            base="ubuntu@22.04",
         )
         await ops_test.model.deploy(
             "mysql-router-k8s",
             application_name=f"router{i}",
             num_units=1,
             channel="8.0/edge",
             trust=True,
+            base="ubuntu@22.04",
         )