Inject GitHub host to be able to clone from another GitHub instance (#922)

* Adding the ability to specify the GitHub Server URL and allowing for it to differ from the Actions workflow host

* Adding tests for injecting the GitHub URL

* Addressing code review comments for PR #922

README.md

@@ -97,6 +97,12 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
     # config --global --add safe.directory <path>`
     # Default: true
     set-safe-directory: ''
+
+    # The base URL for the GitHub instance that you are trying to clone from, will use
+    # environment defaults to fetch from the same instance that the workflow is
+    # running from unless specified. Example URLs are https://github.com or
+    # https://my-ghes-server.example.com
+    github-server-url: ''
 ```
 <!-- end usage -->