Add support for `GITHUB_API_URL` (GHES)

[gr2m]

See https://docs.github.com/en/actions/reference/environment-variables#default-environment-variables.

[oscard0m]

See https://docs.github.com/en/actions/reference/environment-variables#default-environment-variables.

Is it solved by this line or is there something else I'm missing? https://github.com/octokit/action.js/blob/24a544a612c98c85b51ee38d29ca34e638ef09f6/src/index.ts#L35

[gr2m]

Ha! All good then :) But I think we still don't properly test for it, would you like to do that? If not I can create a separate issue