feat(tvix/eval): move shell.nix and .envrc to //tvix

Both //tvix/eval and //tvix/nix_cli have need to for rust tooling available
in $PATH.

Move this one level up, so it's accessible in all subdirectories.

Change-Id: I0763bbe9cefdc962f3a8f86c51e8f67cde8b4b04
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7248
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/.envrc

@@ -0,0 +1,7 @@
+if command -v lorri >/dev/null; then
+  eval "$(lorri direnv)"
+else
+  use nix
+fi
+
+source_up