Override execjs build inputs to avoid building v8

The v8 JS engine doesn't compile successfully on my M1 Mac. However, the
intention was always that execjs would use the runtime provided by
node.js which is already listed as an explicit dependency in shell.nix.

shell.nix

@@ -5,6 +5,11 @@ let
     name = "jamesmead.org-bundler-env";
     inherit ruby;
     gemdir = ./.;
+    gemConfig = pkgs.defaultGemConfig // {
+      execjs = attrs: {
+        propagatedBuildInputs = [];
+      };
+    };
   };
 in stdenv.mkDerivation {
   name = "jamesmead.org";