Disable jewel nightlies

* Jewel currently has serious modem firmware loading issues, it appears
  using the updated 4.13.651 firmware available solves this issue, but
  requires a complete wipe as the partition layout changes. Re-enable
  the nightly builds after this has been sorted out.

Change-Id: I3ee1c496d473d4dc4ea90b0270550688a20dcc1b