This switches tvix-cli over from using `NixCompatIO` to `TvixStoreIO`.
For now, we simply instantiate in-memory services instead of getting
fancy with constructors inside tvix-store, but long-term, we might want
to support some URI syntax, to make this configurable at runtime.
nixpkgs eval tests might be fine (and fast!) with a purely in-memory
backend, but other usages might involve talking to a local tvix-store
over gRPC (using the gRPC client, either unix domain socket or even
further away remote), or running tvix-store in "embedded" mode (using
another client than the gRPC client).
Change-Id: I509afd3dc5ce3f2d52b0fb7067748fab820e26ab
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8572
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
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