Enable custom github server

[jlittle289]

Hard-coded "github.com" URL prevents using this action in enterprise settings or for anyone else with custom github server URLs. Need an option like actions/checkout's github-server-url that lets you define your github host.

[sebastiankugler]

#163 would add this support. However, the author of that PR has provided his own version of this action that supports custom servers: https://github.com/marketplace/actions/ssh-agent-deploy-key. So maybe that action is a better fit for your needs?