Support self-managed instances for agent connect command

In Provide connect command for agent cluster (#463769 - closed) we've introduced in a new modal that shows a glab command to connect to an agent. However, that command doesn't account for self-managed GitLab instances.

To support those the GITLAB_HOST environment variable must be set properly for glab to connect to the proper host.

Implementation guide

1. Update connect_to_agent_modal
2. Add the GITLAB_HOST=${currentHost} part at the beginning of the command in case it's not gitlab.com
3. Update the corresponding tests.