Bootstrap Python 3.6 shards in cron.

[blorente]

Problem

The cron job was not configured to bootstrap the 3.6 version of pants, and therefore when we added the "3.6 + pantsd" shards in #7440, these don't work.

Solution

To add the 3.6 bootstrapping shards to the cron job.

Result

The cron job should now work, maybe?