fix: apply wif identity ref syntax

[rcwbr]

Closes #5

Bug behavior

The GCP auth step in the GitHub Actions workflow shows:

Error: google-github-actions/auth failed with: failed to generate Google Cloud OAuth 2.0 Access Token for [email protected]: {
  "error": {
    "code": 403,
    "message": "Permission 'iam.serviceAccounts.getAccessToken' denied on resource (or it may not exist).",
    "status": "PERMISSION_DENIED",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
        "reason": "IAM_PERMISSION_DENIED",
        "domain": "iam.googleapis.com",
        "metadata": {
          "permission": "iam.serviceAccounts.getAccessToken"
        }
      }
    ]
  }
}

Expected behavior

Successful GCP auth when merging to main.