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/nix_cli/shell.nix

@@ -1,11 +0,0 @@
-let
-  depot = (import ./.. { });
-  pkgs = depot.third_party.nixpkgs;
-
-in
-pkgs.mkShell {
-  buildInputs = [
-    pkgs.rustup
-    pkgs.rust-analyzer
-  ];
-}

tvix/shell.nix

@@ -1,5 +1,6 @@
-{ depot ? import ../../. { }
+{ depot ? import ../. { }
 , pkgs ? depot.third_party.nixpkgs
+, ...
 }:
 
 pkgs.mkShell {