GitHub rate limiting can affect Stack CI

[mpilgrem]

Stack makes unauthenticated requests of the GitHub REST API to access project templates in various GitHub repositories (see Stack.New.settingsFromRepoTemplatePath). GitHub limits those to 60 an hour per user. There is a much higher limit for authenticated requests (which can use 'basic' HTTP authentication), and GitHub provides the GITHUB_TOKEN secret to facilitate authentication in GitHub Actions. Recently, part of Stack's CI (integration tests on Ubuntu only) has begun to fail with rate limit exceeded.

I consider the following be an appropriate way to fix that: Stack would see if certain environment variables were present (I have in mind GH_TOKEN and then GITHUB_TOKEN like the secret - see https://cli.github.com/manual/gh_help_environment) and, if one were present, would seek to use its value to make an authenticated request.

Most users would be oblivious to this feature; not many users will be calling upon a template every minute for an hour. Stack's CI would then set the GH_TOKEN environment variable, using the secret.