test: update: use dvc_repo fixture

To fix test failure.

tests/unit/command/test_update.py

@@ -2,7 +2,7 @@
 from dvc.command.update import CmdUpdate
 
 
-def test_update(mocker):
+def test_update(dvc_repo, mocker):
     targets = ["target1", "target2", "target3"]
     cli_args = parse_args(["update"] + targets)
     assert cli_args.func == CmdUpdate