terraform: Enable usage of the global provider plugin cache

[benjamb]

We have a monolithic repository with 10s of environments. Grouping provider lock updates as a grouped PR therefore makes sense. However, these can time out since dependabot is downloading the providers every time for each directory with a providers lock (even though in many cases they are the same provider at the same version).

Could it make sense to enable the provider plugin cache by setting the TF_PLUGIN_CACHE_DIR environment variable, and then passing -enable-plugin-cache to terraform providers lock to optimise grouped updates?