Revert Vinternal_interpreter_environment tweak

Stefan Monnier pointed out examples like (funcall `(closure
,(let ((cycle (list nil))) (setcdr cycle cycle)) () a)),
where the user can set Vinternal_interpreter_environment
indirectly.
* src/eval.c (Fsetq): Revert recent change, going back to Fassq.
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test-all

00:53:59