[Process] always manually inherit $_SERVER

The $_ENV superglobal is not populated if E is not
present in the variables_order directive. Since
populating this variable is not recommended (for
performance reasons), we should not rely on it.

This change updates the builder so $env=null is
never passed to proc_open(). Instead we always
merge the $_SERVER superglobal into any
environment variables that were manually set on
the builder (unless inherit has been disabled).