Can't install plugins with custom config path after upgrading to Elasticsearch 5.0

Elastic Stack Elasticsearch
1

The ES Plugins Docs state that you can use -Epath.conf=/path/to/custom/config/dir.

From ES_HOME, I've tried the following;

bin/elasticsearch-plugin -Epath.conf=/etc/elasticsearch/custom-conf-dir  list
bin/elasticsearch-plugin -Epath.conf=/etc/elasticsearch/custom-conf-dir/elaticsearch.yml  list

But I always get the following error;

ERROR: E is not a recognized option

I've also tried with the old command line switch, -Des.path.conf=/path/to/custom/config/dir, but that produces a similar error;

ERROR: D is not a recognized option