fix(GHES): respect GITHUB_API_URL when creating installation access token (#38)

Follow up to #36. I just wanted to do some refactoring but turns out I
missed to pass the custom `request` instance to `createAppAuth`. It will
fallback to the default `request` which does not respect
`GITHUB_API_URL`

main.js

@@ -2,9 +2,9 @@
 
 import core from "@actions/core";
 import { createAppAuth } from "@octokit/auth-app";
-import { request } from "@octokit/request";
 
 import { main } from "./lib/main.js";
+import request from "./lib/request.js";
 
 if (!process.env.GITHUB_REPOSITORY) {
   throw new Error("GITHUB_REPOSITORY missing, must be set to '<owner>/<repo>'");