fix(nvd-mirror): use an exponential backoff for retrying pages (#2526)

The NVD API is frequently flaky, and an exponential backoff will give
slightly more time for things to recover before any individual page
fails permanently, which causes the entire mirroring run to abort.

vulnfeeds/cmd/download-cves/main.go

@@ -72,7 +72,7 @@ func downloadCVE2WithOffset(APIKey string, offset int) (page *cves.CVEAPIJSON20S
 	if APIKey != "" {
 		req.Header.Add("apiKey", APIKey)
 	}
-	backoff := retry.NewConstant(6 * time.Second)
+	backoff := retry.NewExponential(6 * time.Second)
 	if err := retry.Do(context.Background(), retry.WithMaxRetries(3, backoff), func(ctx context.Context) error {
 		resp, err := client.Do(req)
 		if err != nil {