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`

lib/request.js

@@ -0,0 +1,8 @@
+import { request } from "@octokit/request";
+
+export default request.defaults({
+  baseUrl: process.env["GITHUB_API_URL"],
+  headers: {
+    "user-agent": "actions/create-github-app-token",
+  },
+});