Enable site isolation for headless shell

[WeizhongX]

Site isolation is disabled by default in headless shell. Enable that through testrunner unless that is explicitly disabled.

[community-tc-integration]

Uh oh! Looks like an error!

Client ID static/taskcluster/github does not have sufficient scopes and is missing the following scopes:

{
  "AnyOf": [
    "queue:rerun-task:taskcluster-github/ZlESXRiRRQmMCspCfS96Jw/G-tz052ERjOnqHDu1Wb3WA",
    "queue:rerun-task-in-project:none",
    {
      "AllOf": [
        "queue:rerun-task",
        "assume:scheduler-id:taskcluster-github/ZlESXRiRRQmMCspCfS96Jw"
      ]
    }
  ]
}

This request requires the client to satisfy the following scope expression:

{
  "AnyOf": [
    "queue:rerun-task:taskcluster-github/ZlESXRiRRQmMCspCfS96Jw/G-tz052ERjOnqHDu1Wb3WA",
    "queue:rerun-task-in-project:none",
    {
      "AllOf": [
        "queue:rerun-task",
        "assume:scheduler-id:taskcluster-github/ZlESXRiRRQmMCspCfS96Jw"
      ]
    }
  ]
}

---

• method: rerunTask
• errorCode: InsufficientScopes
• statusCode: 403
• time: 2024-12-10T16:58:01.931Z

[WeizhongX]

This failure is due to #49545.

@KyleJu @jcscottiii can you help admin merge? Thanks!