Allow the usso token only for the Submitqueue priority requests

[pankti11]

In https://github.com/uber/arcanist/pull/309, we started adding the USSO token to the Submit Queue client to support priority requests. The token was generated using ussh.

After rollout, one service owner reported that arc stack was failing in their service environment because ussh isn’t available outside developer machines or Devpods (e.g., production hosts).

This PR updates the logic to include the USSO token only for Submit Queue requests made via the Priority Queue API (i.e., arc land --skip-submitqueue-checks) to prevent failures in non-dev environments.

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