--es6 switch should take precedence over the KOHA_ELASTICSEARCH variable in .env

I just tried bin/ktd, that's great, I'll be able to get rid of my aliases and run light ktd instances :D

If I add the --es6 switch then I must have KOHA_ELASTICSEARCH=1 to have a working ES in the end.

Without the --es6 switch I must have KOHA_ELASTICSEARCH unset otherwise the koha container crashed on startup when indexing.