allow setting climr cache path via environment variable

[achubaty]

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., https://rstudio.github.io/renv/reference/config.html).

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

[kdaust]

I think that's a great idea. If you're able to put a PR together, that would be much appreciated!