Use DUNE_CACHE_ROOT instead of XDG_CACHE_HOME in toolchains/revstore caches

[ElectreAAS]

This PR is an alternate version of #11612, implementing the 'breaking' option discussed here
It fixes #11584, and helps (but does not fix) #11585.
To recap:

	Current behaviour	With this PR
DUNE_CACHE_ROOT defaults to	XDG/dune/db	XDG/dune/
Build cache	DUNE_CACHE_ROOT or XDG/dune/db	DUNE_CACHE_ROOT/db or XDG/dune/db
Git repo cache	XDG/dune/git-repo only	DUNE_CACHE_ROOT/git-repo or XDG/dune/git-repo
Lmdb cache	XDG/dune/rev_store only	DUNE_CACHE_ROOT/rev_store or XDG/dune/rev_store
Toolchains cache	XDG/dune/toolchains only	DUNE_CACHE_ROOT/toolchains or XDG/dune/toolchains

[rgrinberg]

The implementation is fine but the name db isn't particularly suggestive of anything. I'd suggest a name like rules or build.

Also, it would be good to have a test that just prints all o these paths with/without configuration.

[ElectreAAS]

The implementation is fine but the name db isn't particularly suggestive of anything. I'd suggest a name like rules or build.

Gotta say I disagree here. Sure the current name isn't the best, but changing that would mean breaking everyone's cache, not just people who have DUNE_CACHE_ROOT set. The previous breakage was deemed acceptable because we assumed not that many people have that variable set (and that's not completely true anyway), but here it almost seems intentional, like we break people's caches just to mess with them.

Also, it would be good to have a test that just prints all o these paths with/without configuration.

Sure!

[rgrinberg]

When you say break, you mean we force people to repopulate the cache by rerunning the rules? If so, then this is something that happens regularly.

You can keep the current name if you prefer though. All of this is fairly invisible to users anyway.

[ElectreAAS]

Also, it would be good to have a test that just prints all o these paths with/without configuration.

There is now test/blackbox-tests/test-cases/dune-cache/possible-locations.t

[Alizter]

Let's move that into the enabled branch

[ElectreAAS]

I'm aware this needs not just a changelog entry (which it has), but a mention in the actual docs. I put off making that section until the back-and-forth between this PR and #11612 was well and truly ended, and so it will land in a subsequent PR.