feat(tvix/nix-daemon): Initial skeleton for nix-daemon

The goal is to create a drop-in replacement nix-daemon that nix-cpp can
use as a `daemon` store.

Change-Id: Ie092047dcc6a24a3b8d8d1b808f3e6fd2c493bf2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12711
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-daemon/default.nix

@@ -0,0 +1,5 @@
+{ depot, ... }:
+
+depot.tvix.crates.workspaceMembers.nix-daemon.build.override {
+  runTests = true;
+}