feat: Allow user to set the cache directory

[ruben-arts]

Closes #681

This adds the ability to set the cache dir using an environment variable:

export PIXI_CACHE_DIR=/path/to/cache
# OR 
export RATTLER_CACHE_DIR=/path/to/cache

I added the RATTLER_CACHE_DIR so that we could use that in all our libraries so that it gets shared (like it does before this change)
I added the PIXI_CACHE_DIR to not confuse users that don't have any knowledge that rattler is our backend.

[amirhosseindavoody]

Thanks for adding this feature.

Is there a central list of all pixi related environment variables?

[ruben-arts]

Thanks for adding this feature.

Is there a central list of all pixi related environment variables?

No sorry, the docs could use some extra love

[amirhosseindavoody]

I'll see if I can add the doc page.

Is there a guideline for documentation contributions?

[ruben-arts]

Cool, no it's mostly me writing down what I can think of. Some outsider help would be awesome. Open to ideas!