Allow adding paths to cliPluginsExtraDirs via env var

[leighmcculloch]

Description

When setting up some systems, and in dotfiles, it would be really helpful if it was possible to add paths to the cliPluginsExtraDirs config via an environment variable, similar to how the PATH variable works.

It isn't often easy to modify the .docker/config.json file, or store it premade, since so many different parameters find their way into the config.

It'd be amazing if we could add paths by doing:

export DOCKER_CLI_PLUGINS_EXTRA_DIRS="$DOCKER_CLI_PLUGINS_EXTRA_DIRS:/home/username/my/path"