fix context export not working if conflicts with instance variable

[tanhauhau]

sorry, my bad for causing this regression bug.

Fix #3983

unknowingly, module_js and instance_js uses the same var_lookup. and instance_js var will overwrite module_js after traversing it's ast.

previously it works because, the module_js ast is traversed and extract exports. before traversing instance ast (which overwrites the module_js's vars)