Use mozconfig of the failed task

We should use the mozconfig of the task that fails. @padenot suggested this as an example:

async def mozconfig_for_task(task_id, source_repo):
      task = await fetch_json(
          f"[https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/{task_id}](https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/%7Btask_id%7D)"
      )

      env = task["payload"].get("env", {})
      extra = json.loads(env.get("EXTRA_MOZHARNESS_CONFIG", "{}"))

      if env.get("MOZCONFIG"):
          return source_repo / env["MOZCONFIG"]

      configs = load_mozharness_configs(
          env["MOZHARNESS_CONFIG"],
          env.get("MOZHARNESS_CONFIG_PATHS", ""),
          source_repo,
      )

      config = merge_configs(configs)
      config.update(extra)

      if "src_mozconfig" in config:
          return source_repo / config["src_mozconfig"]

      return (
          source_repo
          / config["app_name"]
          / "config"
          / "mozconfigs"
          / config["mozconfig_platform"]
          / config["mozconfig

Start by tracing the failed-task handling and the existing mozconfig selection logic. Use the proposed mozconfig_for_task entry point and inspect the task payload's env fields, including MOZCONFIG, MOZHARNESS_CONFIG, and EXTRA_MOZHARNESS_CONFIG. Done means the failed task's mozconfig is selected consistently.