lib/types.nix: fix missing inherit

I think there was a silent (i.e. semantic) merge conflict between PR #101139 and
PR #100456.  This commit should fix the error, which manifests as follows:

  error: undefined variable 'boolToString' at /home/kkini/src/nixpkgs/lib/types.nix:552:42

lib/types.nix

@@ -57,6 +57,9 @@ let
     escapeNixString
     isCoercibleToString
     ;
+  inherit (lib.trivial)
+    boolToString
+    ;
 
   inherit (lib.modules) mergeDefinitions;
   outer_types =