test(expo): Pin expo publisher container to node 12
test(expo): Pin expo publisher container to node 12

The version of expo-cli we're using is not compatible with node 14
and the LTS container recently bumped to that version, causing CI
to fail.

bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js