feat: make builtins.currentSystem illegal in depot

Since cl/7260 has eliminated all uses of builtins.currentSystem
in the main evaluation path of depot, we can ensure that we use
localSystem consistently in the future by making it impossible
to access the value of builtins.currentSystem in readTree nodes.

Change-Id: I6ded54021c42fcf31a80268149179f95f22bad88
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7261
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

default.nix

@@ -62,6 +62,9 @@ let
     filter = parts: args: corpFilter parts (usersFilter parts args);
     scopedArgs = {
       __findFile = _: _: throw "Do not import from NIX_PATH in the depot!";
+      builtins = builtins // {
+        currentSystem = throw "Use localSystem from the readTree args instead of builtins.currentSystem!";
+      };
     };
   };