feat: set default baseUrl to allow GHES usage

[erodewald]

Using the built-in environment variable for the GitHub URLs, we can allow support for any GitHub instance. Works well in my GHES instance.

• [x] Added optional input to override URL
• [x] Updated readme.md
• [x] Updated action.yml
• [x] Updated tests

Resolves #56

• [x] npm install
• [x] npm test
• [x] npm run lint
• [x] npm run format
• [x] npm run lint-fix

[github-actions[bot]]

:tada: This PR is included in version 1.6.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: