This reverts commit d9a7d03da8.
Reason for this is that it actually doesn't migitate the issue on nix
stable for another reason: builtins.tryEval doesn't prevent the error
generated by builtins.functionArgs from halting evaluation:
> builtins.tryEval (builtins.functionArgs builtins.functionArgs)
error: 'functionArgs' requires a function, at (string):1:19
Thus it seems that there is no workaround to make
lib.generators.toPretty work with nix stable and primops since there is
no way to distinguish between primops and lambdas in nix.
06d3b28987