Toolchains cache does not follow DUNE_CACHE_ROOT

[emillon]

Hi,
The toolchains cache is at the moment separate from the location of the shared dune cache.
This can lead to confusing behavior.

For example, suppose if a CI configures a custom DUNE_CACHE_ROOT mounted to a persistent location. They'll always have to start building a compiler because they're not restored from the main cache. It's possible that this cache miss cascades because of #11583 too.

I don't have a concrete solution for that; maybe if DUNE_CACHE_ROOT it should store the toolchains info there instead of a generic XDG solution.

cc @mefyl