Use GitHub App for context auth instead of PAT

[Eldarrin]

What is the problem you're trying to solve
When using a private org for GitHub you can block PAT tokens outside the network and allow Apps only, or you could just prefer the better security for GitHub Apps.

Currently it seems the builder only support PAT for private repo auth.

Describe the solution you'd like
An alternative to --git-access-token that allows the use of a GitHub App for auth to download source and/or hooks up to events.

Additional context
https://docs.github.com/en/apps