feat: make eval store configurable

[fricklerhandwerk]

evaluations write garbage, which we don't use, to disk if we hit the physical store, so this should speed things up a bit

[RaitoBezarius]

TODO to unblock:

• add tmpfs creation in the nixos modules
• configure that evaluation store dir
• configure it for the staging deployment

[Erethon]

Something to keep in mind if we're to implement this (and we might need to document it even if we keep using /nix/store): The tracker produces a lot of derivations, currently in the production instance we have ~1.3 million files in /nix/store and that's only with 24.11 and part of 24.05 ingested. This makes it easy for a filesystem to run out of inodes (it happened earlier in the production instance and I had to intervene).

A tmpfs with default settings has number_of_pages / 2 inodes. This means that on a typical system with 16GB of memory and a pagesize of 4096, we get ~4 million pages, which means ~2 million inodes. Two million inodes might not be enough for our usecase, so we'll have to mount the tmpfs with more inodes instead of the default.

Then again assuming a derivation is ~3kB in size (which I think is fairly typical) and we have ~1.5m of those, that's already almost 5GB of data, so we'll face memory issues as well as inode issues. If we really don't need those derivations, we should have a way to clean them as soon as we're done with them. And ideally not have them touch the disk but use a tmpfs, as per the original idea.