chore(tvix/eval): add simple .envrc / shell.nix

Change-Id: I10a44b92391a8ac4d7ec5e4b878953c215f59908
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6620
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/.envrc

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

tvix/eval/shell.nix

@@ -0,0 +1,12 @@
+{ depot ? import ../../. { }
+, pkgs ? depot.third_party.nixpkgs
+}:
+
+pkgs.mkShell {
+  name = "tvix-eval-dev-env";
+  packages = [
+    pkgs.cargo
+    pkgs.rustc
+    pkgs.clippy
+  ];
+}