Fix `clear_cache_on_start` in pystencils.json

Due two weird recursive dependencies `clear_cache` will not be
available at this point. In-lining this function will make
the config key `cache.clear_cache_on_start: true` work again.
13 jobs for fix-clear-cache-on-startup in 20 minutes and 40 seconds (queued for 2 minutes and 36 seconds)