Improve robustness of stack get USNs action

[robdimsdale]

In the action that retrieves Stack USNS, we have retry handling with exponential backoff for HTTP errors, but occasionally we see other types of errors that likely would succeed on retry (e.g. Get "https://ubuntu.com/security/notices/rss.xml": net/http: TLS handshake timeout).

We should include these types of errors in the retry handling, so as to improve the robustness of the action.

As an implementation note, the underlying library defines HTTPError but not other errors (e.g. transport errors like the one above). So we'd probably have to do the type assertion (errors.As()) on the net/http error type.