Either the environment variable TF_PLUGIN_CACHE_DIR or a setting in the
CLI config file (~/.terraformrc on Unix) allow opting in to the plugin
caching behavior.

This is opt-in because for new users we don't want to pollute their system
with extra directories they don't know about. By opting in to caching, the
user is assuming the responsibility to occasionally prune the cache over
time as older plugins become stale and unused.