Allow environment variable NTERACT_DESKTOP_DISABLE_AUTO_UPDATE to disable auto-update.
Allow environment variable NTERACT_DESKTOP_DISABLE_AUTO_UPDATE to disable auto-update.

Fixes #3517

nteract/nteract,nteract/nteract,nteract/composition,rgbkrk/nteract,nteract/nteract,rgbkrk/nteract,nteract/composition,nteract/nteract,rgbkrk/nteract,nteract/composition,nteract/nteract,rgbkrk/nteract,rgbkrk/nteract