Cancel `Internal Tests` private job if the public job is cancelled

[bfops]

When a public Internal Tests job is cancelled, cancel the corresponding private job as well.

TODO

• Split the job into a part that kicks off the workflow, and one that waits for it
• Do something like this to cancel the job that was kicked off:

  - name: Run only if job was cancelled
    if: ${{ cancelled() }}
    run: echo "Job was cancelled!"