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/default.nix

@@ -0,0 +1,2 @@
+{...}:
+{}