net/rabbitmq: Fix rabbitmq rc.d script to use %%_ERLANG_LIB%%

[Why]
In commit c9033971719dc098b34276f11b8e5a5e22b966fd, the variable was
renamed from ERLANG_LIB to _ERLANG_LIB. However the rc.d script was
not updated to reflect that change.

Therefore the script didn't use the expected version of Erlang. This
meant that it could use lang/erlang if it was installed (which could
be incompatible) or not start at all.

[How]
The rc.d script is simply updated to use the new variable name. The port
revision is bumped to note the change.

PR: 271269
Reported by: Miroslav Lachman <000.fbsd@quip.cz>