Fix calling condition for Arbiter.log.reopen_files()

`GUNICORN_FD` is not always set in a `reexec`ed child process.
Considering `Arbiter.master_pid` is set after log reopening,
choose `GUNICORN_PID`.

gunicorn/arbiter.py

@@ -93,7 +93,7 @@ def setup(self, app):
             self.log = self.cfg.logger_class(app.cfg)
 
         # reopen files
-        if 'GUNICORN_FD' in os.environ:
+        if 'GUNICORN_PID' in os.environ:
             self.log.reopen_files()
 
         self.worker_class = self.cfg.worker_class