lei: ensure --stdin sets %ENV and $current_lei

lei: ensure --stdin sets %ENV and $current_lei

--stdin usage means the current request can be delayed
indefinitely while other requests with different %ENV
come in.  So make sure our warnings and %ENV can match
non-stdin behavior.

This probably fix segfaults during process cleanup on OpenBSD
since _lei_atfork_child use non-localized assignment of
$current_lei.  But it could be another red herring.  Either way,
it's the right thing to do from an environment replication
perspective.