[AIRFLOW-XXX] Replaces incorrect env var name in INSTALL (#3788)
[AIRFLOW-XXX] Replaces incorrect env var name in INSTALL (#3788)

INSTALL mis-named AIRFLOW_GPL_UNIDECODE environment variable.
owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow