feat(tvix): set up cargo rust project

First steps for baba

Change-Id: Id6a68c5630cb85f280f4dcc7b2acf10c02454fd6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4732
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/shell.nix

@@ -0,0 +1,10 @@
+let
+  depot = (import ./.. {});
+  pkgs = depot.third_party.nixpkgs;
+
+in pkgs.mkShell {
+  buildInputs = [
+    pkgs.rustup
+    pkgs.rust-analyzer
+  ];
+}