Allow passing base url for GitHub Enterprise Server

[armaandhull]

This action currently only works with public github repos. Octokit allows passing a baseUrl for repos hosted on GitHub Enterprise Server, can we modify this action to work with GHE?

https://github.com/octokit/octokit.js/blob/main/README.md#constructor-options

Cc @pascalgn

[armaandhull]

Opened a PR for this as well #40