[Process] fix issue where env contains array vals

See ticket #7196
There are cases where $env or $_ENV can contain a value that is an array
This will cause Process to throw an Array to String conversion exception
Initially I submitted a patch of Process.php, however Fabien indicated
that it shouldn't be fixed there (see pull request #7134).  Before a
simple work around would be register_argc_argv = On in php.ini, however
with recent changes to symfony, this seems to no longer work.