seek(0) before sys.stdout.truncate()

Without this we only truncate to where the file was when we did the
stdout/err redirect.

What I observed was it truncated out the "journal locking" output, but
not anything from a prior run of the program.

EDMarketConnector.py

@@ -242,6 +242,8 @@ if __name__ == '__main__':  # noqa: C901
 
     if getattr(sys, 'frozen', False):
         # Now that we're sure we're the only instance running we can truncate the logfile
+        logger.trace('Truncating plain logfile')
+        sys.stdout.seek(0)
         sys.stdout.truncate()