feat(tvix/cli): use TvixStoreIO instead of NixCompatIO

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

tvix/Cargo.lock

@@ -2640,6 +2640,7 @@ dependencies = [
  "ssri",
  "thiserror",
  "tvix-eval",
+ "tvix-store",
  "wu-manber",
 ]