Bug: App auth path matching not work for Enterprise

[yanyongyu]

          This change breaks github enterprise, because urls begin with /api/v3, this regex no longer matches for the /app endpoint

Originally posted by @brainiac in #42 (comment)

[yanyongyu]

octokit matches the path by replacing the base url first:

https://github.com/octokit/auth-app.js/blob/c0068e06081a5d930799285a7c79c9c948b676b6/src/hook.ts#L43