Support GitHub Enterprise

[zachwhaley]

GitHub Actions sets the environment variable GITHUB_API_URL to the API URL of the instance of GitHub. For example, if running on github.com, then GITHUB_API_URL will be https://api.github.com; if running on ghe.my-company.com, then GITHUB_API_URL will be https://ghe.my-company.com/api/v3.

By setting the baseUrl of Octokit to the GITHUB_API_URL environment variable, any instance of GitHub Actions is supported, github.com or GitHub Enterprise.