Flaky test (deadlock?) in debusine/tasks/tests/test_task.py::RunCommandTaskTests

Test jobs where this has been observed so far:

• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_execute_cmd_cancelled_killed_with_sigterm
• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_execute_cmd_cancelled_killed_with_sigterm
• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_execute_cmd_cancelled_killed_with_sigterm
• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_execute_cmd_cancelled_killed_with_sigkill
• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_execute_cmd_finished_before_killpg_sigkill
• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_command_finish_with_failure
• debusine/tasks/tests/test_task.py::RunCommandTaskTests::test_execute_cmd_cancelled_killed_with_sigterm