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/.envrc

@@ -0,0 +1,10 @@
+source_env ../.envrc
+
+if type lorri &>/dev/null; then
+    echo "direnv: using lorri from PATH ($(type -p lorri))"
+    eval "$(lorri direnv)"
+else
+    # fall back to using direnv's builtin nix support
+    # to prevent bootstrapping problems.
+    use nix
+fi