Fix SECRET_KEY autogeneration in Debian package

debian/contrib/uffd-nginxauth.cfg

 FLASK_ENV="production"
-#SECRET=autogenerated by postinst script
+#SECRET_KEY=autogenerated by postinst script
 
 # URLs of the OAuth2-based identity provider (must be set!)
 #OAUTH2_AUTH_URL = 'http://localhost:5001/oauth2/authorize'