Issue: The part of the action that creates installation token does not work behind a proxy.

Reason: That part uses the @Octokit/request library to make a request to the access_token endpoint. That library in turn uses the library noda-fetch without sending in any proxy agent. The noda-fetch library does not check for http_proxy environment variables (node-fetch/node-fetch#195).

The call that fetches repo installation uses Octokit from the @actions/github library, which supports running behind a proxy.

Fix: Use @actions/github also for creating access token.

I have a simple fix for this but I need write access to be able to contribute.