nix: ccache sets CCACHE_BASE_DIR instead of CCACHE_BASEDIR

[ConnorBaker]

Firstly thank you @Kha for setting up Lean so it uses CCache, even with Nix. I'm currently cribbing some of it to try to speed up the CUDA work I'm doing.

In looking at

https://github.com/leanprover/lean4/blob/869d64e97ac5acf557c949c2af917db633999f8d/nix/packages.nix#L14C26-L14C26

I noticed that the environment variable being set isn't in the current docs:

https://ccache.dev/manual/4.8.2.html#_configuration_options

They have CCACHE_BASEDIR instead of CCACHE_BASE_DIR. Looking at the docs, it looks like most of the environment variables are CCACHE_* where * doesn't have any underscores or hyphens. There are a few exceptions, but I don't know if there's a general rule.

Any ideas if this breaks caching, or if the docs are wrong?

[Kha]

The Nix build has been officially deprecated #4895