lib/gvariant: Fix error message for unsupported primitives

Without this, passing an integer to a setting will fail with a confusing error:

    error: cannot coerce an integer to a string

lib/gvariant.nix

@@ -71,7 +71,7 @@ rec {
     else if isGVariant v then
       v
     else
-      throw "The GVariant type of ${v} can't be inferred.";
+      throw "The GVariant type of “${builtins.typeOf v}” can't be inferred.";
 
   /* Returns the GVariant array from the given type of the elements and a Nix list.