internal/contributebot: factor out *gitHubInstallAuth.fetchToken method (#1046)
internal/contributebot: factor out *gitHubInstallAuth.fetchToken method (#1046)

Makes RoundTrip easier to understand, but also allows the cached token
to be reused for other purposes (like Git tool access).

Updates #687
google/go-cloud,google/go-cloud,google/go-cloud