Allow `environment-cache` to take an environment as an argument.

Currently, if you want to use this API endpoint to flush the environment cache, you have no choice but to flush the cache for ALL environments. Ideally, it would take an environment as parameter, so that you can only flush the environments you know to have changed.

As it stands, using this API endpoint is basically hitting the "slow down the master" button, if it's a master under heavy use.