rabbitmq-env.bat: Define `$RABBITMQ_QUORUM_DIR`

The variable was used in `rabbitmq-server.bat` & friends but not
defined, leading to a broken RabbitMQ on Windows.

Here is the crash logged early in RabbitMQ startup:

    2018-10-31 10:34:23.672 [error] <0.42.0> application_controller: unterminated string starting with "\\=/RABBITMQ_SERV": "\\=/RABBITMQ_SERVER_START_ARGSRABBITMQ_DIST_ARGSTARVAR
    2018-10-31 10:34:23.672 [error] <0.7.0>
    Error description:
        init:do_boot/3 line 819
        init:start_em/1 line 1111
        rabbit:start_it/1 line 460
        rabbit:broker_start/0 line 336
        rabbit:start_apps/2 line 518
        app_utils:load_applications/1 line 41
        app_utils:load_applications/2 line 110
    throw:{error,{bad_environment_value,"\"\\\\=/RABBITMQ_SERVER_START_ARGSRABBITMQ_DIST_ARGSTARVAR"}}

0 files changed, 0 insertions, 0 deletions