json: don't spoil instance with per-call options

When json.decode is used with 2 arguments, 2nd argument seeps out to the
json configuration of the instance. Moreover, due to current
serializer.cfg implementation it remains invisible while checking
settings using json.cfg table.

This fixes commit 6508ddb7 ('json: fix
stack-use-after-scope in json_decode()').

Closes #4761