nixos/lib/test-driver: fix linting after compatibility clean‐up

The previous commit removed the handling of `dict` arguments, but
didn’t adjust the type, leading to the following type-checking error:

    test_driver/driver.py:216: error: Argument 1 to "NixStartScript" has incompatible type "str | dict[Any, Any]"; expected "str"  [arg-type]

It also left an unused import that Ruff is unhappy about:

    build/lib/test_driver/driver.py:11:22: F401 [*] `colorama.Fore` imported but unused
    …
    build/lib/test_driver/driver.py:11:28: F401 [*] `colorama.Style` imported but unused

Fixes: 71306e6

nixos/lib/test-driver/test_driver/driver.py

@@ -8,8 +8,6 @@
 from pathlib import Path
 from typing import Any
 
-from colorama import Fore, Style
-
 from test_driver.logger import AbstractLogger
 from test_driver.machine import Machine, NixStartScript, retry
 from test_driver.polling_condition import PollingCondition
@@ -206,7 +204,7 @@ def terminate_test(self) -> None:
 
     def create_machine(
         self,
-        start_command: str | dict,
+        start_command: str,
         *,
         name: str | None = None,
         keep_vm_state: bool = False,