test: properly mock the environment (#1385)

These two tests change `os.environ` without restoring it afterwards,
which means that the changes leak into other tests. Change that by using
`monkeypatch.setenv`.

test/test_model.py

@@ -3246,7 +3246,7 @@ def test_relation_remote_app_name_env(self, monkeypatch: pytest.MonkeyPatch):
         monkeypatch.setenv('JUJU_RELATION_ID', 'x:5')
         monkeypatch.setenv('JUJU_REMOTE_APP', 'remoteapp1')
         assert self.backend.relation_remote_app_name(5) == 'remoteapp1'
-        os.environ['JUJU_RELATION_ID'] = '5'
+        monkeypatch.setenv('JUJU_RELATION_ID', '5')
         assert self.backend.relation_remote_app_name(5) == 'remoteapp1'
 
     def test_relation_remote_app_name_script_success(
@@ -3270,7 +3270,7 @@ def test_relation_remote_app_name_script_success(
 
         # JUJU_RELATION_ID unset but JUJU_REMOTE_APP set
         monkeypatch.delenv('JUJU_RELATION_ID')
-        os.environ['JUJU_REMOTE_APP'] = 'remoteapp1'
+        monkeypatch.setenv('JUJU_REMOTE_APP', 'remoteapp1')
         assert self.backend.relation_remote_app_name(5) == 'remoteapp2'
 
         # Both set, but JUJU_RELATION_ID a different relation