process_stub_unix: fix race condition with reading of env file

The stub incorrectly send its pid before reading the environment file,
and qtcreator does interpret that as sign that it can delete the
environment file.

Task-number: QTCREATORBUG-9024
Change-Id: Id44777abdfdd94aa10815cb00ca27846b95a3ec7
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>