default.nix: warn the user if they try to build the entire scope

The `AAAAAASomeThingsBuildTooMuch` attrname is borrowed from a
similar attribute in nixpkgs' `top-level/all-packages.nix`.

src/default.nix

@@ -9,6 +9,12 @@ let
 
   ownerboot = { overlays }: let
     base = final: prev: {
+      AAAAAASomeThingsBuildTooMuch = throw ''
+        Please don't try to `nix-build` the entire ownerboot scope;
+        there's a lot of stuff in it that need not be built for any
+        particular target.  You probably want to add `.main` to your
+        `nix-build -A` argument.
+      '';
       inherit hostPlatform;
       nixpkgsOnBuildForHost = pkgsFun
         (nixpkgsArgs