Provide a way to fix `are you getting rate limited by Github?`

[JounQin]

For example, support environment variable GITHUB_TOKEN.

[JounQin]

OK, I find out --github-api-token option, this should be mentioned in the error logging.

[afbjorklund]

The enviroment variable is called MACHINE_GITHUB_API_TOKEN (same as the option)

[JounQin]

@afbjorklund

Thanks, so the error message can be improved in this case.