fixed paths and env variable name for plugings

[Mandraenke]

This may fix #1352

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

Tried to fix enabling community plugins, downloaded via init-containers.

Changed PLUGIN_DIR to RABBITMQ_PLUGIN_DIR and added the default path of shipped plugins to its value.

Additional Context

Local Testing

Please ensure you run the unit, integration and system tests before approving the PR.

To run the unit and integration tests:

$ make unit-tests integration-tests

You will need to target a k8s cluster and have the operator deployed for running the system tests.

For example, for a Kubernetes context named dev-bunny:

[~]$ kubectl exec -ti rabbitmq-server-0 -- bash
Defaulted container "rabbitmq" out of: rabbitmq, copy-community-plugins (init), setup-container (init)
I have no name!@rabbitmq-server-0:/$ rabbitmq-plugins directories
Listing plugin directories used by node [email protected]
Plugin archives directory: /opt/rabbitmq/plugins:/opt/rabbitmq/community-plugins:/opt/bitnami/rabbitmq/plugins
Plugin expansion directory: /bitnami/rabbitmq/mnesia/[email protected]
Enabled plugins file: /operator/enabled_plugins
I have no name!@rabbitmq-server-0:/$ rabbitmq-plugins list
Listing plugins with pattern ".*" ...
 Configured: E = explicitly enabled; e = implicitly enabled
 | Status: * = running on [email protected]
 |/
[  ] rabbitmq_amqp1_0                  3.10.11
[  ] rabbitmq_auth_backend_cache       3.10.11
[  ] rabbitmq_auth_backend_http        3.10.11
[  ] rabbitmq_auth_backend_ldap        3.10.11
[  ] rabbitmq_auth_backend_oauth2      3.10.11
[  ] rabbitmq_auth_mechanism_ssl       3.10.11
[  ] rabbitmq_consistent_hash_exchange 3.10.11
[E*] rabbitmq_delayed_message_exchange 3.10.2
[  ] rabbitmq_event_exchange           3.10.11
[  ] rabbitmq_federation               3.10.11
[  ] rabbitmq_federation_management    3.10.11
[  ] rabbitmq_jms_topic_exchange       3.10.11
[E*] rabbitmq_management               3.10.11
[e*] rabbitmq_management_agent         3.10.11
[  ] rabbitmq_mqtt                     3.10.11
[  ] rabbitmq_peer_discovery_aws       3.10.11
[e*] rabbitmq_peer_discovery_common    3.10.11
[  ] rabbitmq_peer_discovery_consul    3.10.11
[  ] rabbitmq_peer_discovery_etcd      3.10.11
[E*] rabbitmq_peer_discovery_k8s       3.10.11
[E*] rabbitmq_prometheus               3.10.11
[  ] rabbitmq_random_exchange          3.10.11
[  ] rabbitmq_recent_history_exchange  3.10.11
[  ] rabbitmq_sharding                 3.10.11
[  ] rabbitmq_shovel                   3.10.11
[  ] rabbitmq_shovel_management        3.10.11
[  ] rabbitmq_stomp                    3.10.11
[  ] rabbitmq_stream                   3.10.11
[  ] rabbitmq_stream_management        3.10.11
[  ] rabbitmq_top                      3.10.11
[  ] rabbitmq_tracing                  3.10.11
[  ] rabbitmq_trust_store              3.10.11
[e*] rabbitmq_web_dispatch             3.10.11
[  ] rabbitmq_web_mqtt                 3.10.11
[  ] rabbitmq_web_mqtt_examples        3.10.11
[  ] rabbitmq_web_stomp                3.10.11
[  ] rabbitmq_web_stomp_examples       3.10.11
I have no name!@rabbitmq-server-0:/$ 

[Zerpet]

I'm happy with the change. It does not impact non-Bitnami packaged Operators, and it fixes the bitnami-packaged Operators.

[Zerpet]

CI fails because the workflow requires a GitHub Secret that is not passed since the PR comes from a fork. We can merge anyway.