Add rebar3_hex plugin reference to rebar3.config generated by erlang.mk

[deadtrickster]

This allows erlang.mk hex publish target to progress beyond ===> Command hex not found error

[gerhard]

Forgot to add the backport v3.9.x label before closing this. Doing it manually now, but will remember to add it next time.

[gerhard]

Backported to v3.9.x & v3.8.x

[michaelklishin]

We should perhaps upstream this? @dumbbell @lhoguin

[deadtrickster]

Turns out we don't use bare rebar3, but our own wrapper - hexpm-cli, It has hardcoded config with rebar3_hex. I created PR to upgrade it to recent rebar3 with OTP 24 support - rabbitmq/hexpm-cli#2

[lhoguin]

For the record: no this kind of change should not be added to Erlang.mk. Erlang.mk produces a rebar.config file that's meant to be used for projects used as dependencies. There is no problem in that case because the plugins can be added to the top-level rebar.config file.

[dumbbell]

Also, erlang.mk is a generated file. We should revert this change now instead of being caught by surprise with some obscure regressions when we update Erlang.mk.