Improve GITHUB_API_URL default value handling

[tdabasinskas]

GitHub Actions already has GITHUB_API_URL environment variable which contains GitHub API URL. We should be smart about overriding it and, if it exists and already has value, we should not override it.

In addition, we should strip the last trailing slash (if it exists) from GITHUB_API_URL, as API calls already append the slash.

This change allows running the action on GitHub Enterprise without the need to explicitly set GITHUB_CUSTOM_API_URL.

Proposed Changes

1. If GITHUB_CUSTOM_API_URL exists, use its value as API URL
2. If GITHUB_CUSTOM_API_URL does not exist, but GITHUB_API_URL is already set, use its value as API URL
3. If neither GITHUB_CUSTOM_API_URL nor GITHUB_API_URL is set, use https://api.github.com as API URL
4. Trailing slash is always trimmed from GITHUB_API_URL.

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request

Reviewing Maintainer

• Label as breaking if this is a large fundamental change
• Label as either automation, bug, documentation, enhancement, infrastructure, or performance