allow setting climr cache path via environment variable

Setting the cache path via option `climr.cache.path` is useful for configuring e.g. a shared network-accessible location to store cached data, but it requires setup for each project.

Allowing the use of an environment variable (e.g., `CLIMR_CACHE_PATH`) lets a system administrator set this at the machine level via `.Renviron`, speeding up users' analyses once the network cache is built, and reducing network and storage pressure.

I would expect that the environment variable gets used if set, but if the option is set, the option takes precedence (see e.g., ).

If you are open to a PR for this, I can prepare one.