chore(tvix): move nix-store CLI scaffolding to subfolder

For some reason a top-level Rust project ended up in this location,
which is incompatible with the actual project structure that's being
prepared for merge right now.

Change-Id: I9d919ad72fc7e4e4d8cbb9899e7f8d90fa7ca87a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6060
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>

tvix/nix_cli/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, tvix!");
+}