feat(tvix/cli): implement NixCompatIO helper type

This type allows for temporarily compatibility with the C++ Nix store,
specifically (for now) it gives us the store directory used by Nix and
imports files the same way.

Change-Id: I4767794ef2863eba49661315c63c4e17de946d60
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7587
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -4229,6 +4229,10 @@ rec {
             name = "rustyline";
             packageId = "rustyline";
           }
+          {
+            name = "smol_str";
+            packageId = "smol_str";
+          }
           {
             name = "tvix-eval";
             packageId = "tvix-eval";