Fix an escaping error.

Without this one-character change, we're checking the exit code of
the invoking shell, not the one running the command.

Author: mreid-moz

provisioning/aws/launch_worker.py

@@ -71,7 +71,7 @@ def get_user_data(self):
 set +e
 echo "Beginning job $JOB_NAME ..." >> "$LOG"
 $MAIN &>> "$LOG"
-JOB_EXIT_CODE=$?
+JOB_EXIT_CODE=\$?
 echo "Finished job $JOB_NAME" >> "$LOG"
 set -e
 echo "'$MAIN' exited with code \$JOB_EXIT_CODE" >> "$LOG"