fix scripts/update to not fail because of linuxPackages

impure.nix

@@ -63,8 +63,20 @@ let
 
   pkgs = mkPkgs {};
 
-  subAttrs = attrs: lib.filterAttrs (name: value: builtins.isAttrs value) attrs;
+  # subAttrNames :: AttrSet -> [ String ]
+  # returns the names of all items in `attrs` which are themselves attrsets.
+  # presumably, this is the list of items which we may wish to descend into.
   subAttrNames = attrs: builtins.attrNames (subAttrs attrs);
+  subAttrs = attrs: lib.filterAttrs
+    (name: value:
+      let
+        # many attributes in the `pkgs` set do not evaluate (even `pkgs.sane`, thanks to e.g. linuxPackages).
+        # wrap in `tryEval` to deal with that, and not descend into such attributes.
+        isAttrs = builtins.tryEval (builtins.isAttrs value);
+      in
+        isAttrs.success && isAttrs.value
+    )
+    attrs;
 
   # given the path to a package, and that package, returns a list of all attr-paths (stringified)
   # which should be updated as part of that package (including the package in question).