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?

action.js/src/index.ts

Line 35 in 24a544a

return process.env["GITHUB_API_URL"] || "https://api.github.com";

[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