fix(cli,install): authenticity of host 'github.com' can't be established

[geoffreyvanwyk]

When using the install_plugins command noninteractively on automation platforms such as GitHub Actions or Ansible, it can sometimes hang and never complete. When I ran the command interactively, it prompted me to either accept or reject to the host key of an unknown host.

The authenticity of host 'github.com (20.87.245.0)' can't be established.
ED25519 key fingerprint is SHA256:+DiY3wvvV6TuJJhbpZisF/zLDA0zPMSvHdkr4UvCOqU.
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])?

I found out the cause was the plugin https://github.com/alexwforsythe/tmux-which-key having a git submodule with an SSH URL: https://github.com/alexwforsythe/tmux-which-key/blob/main/.gitmodules . For open source projects, this is an error.

This pull request proposes to exit the install_plugins command immediately if the host is unknown.

Resolves #219