Add new `PINS_CACHE_PATH` env var, plus documentation

[juliasilge]

Closes #747

With these changes, we can now use an environment variable to specify where the cache goes:

withr::local_envvar(list("PINS_CACHE_PATH" = "~/Desktop"))
server <- pins:::rsc_server(auth = "envvar")
pins::board_cache_path(paste0("connect-", rlang::hash(server$url)))
#> ~/Desktop/connect-31a399613ac85efda77ffe6ff20c105a

^{Created on 2023-06-02 with reprex v2.0.2}

This also finally fixes #611 by documenting how the env variables work, with some advice on a read-only filesystem like AWS Lambda.

[machow]

I just double checked in pins-python, and apparently it uses PINS_CACHE_DIR as an optional env var for overriding the cache directory. (I don't think this is documented, and probably exists for unit tests..). May be useful to change here or there to match env vars?

https://github.com/rstudio/pins-python/blob/7ded06943c6ea3236e5da4443a3869073d66aeb1/pins/config.py#L8

[juliasilge]

Thanks to both!

@machow I changed the env var here so it matches what pins for Python has. 👍

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