cve-update-nvd2-native: increase retry count

Current 503 errors seem to last several seconds.
In most cases there are two errors and third request succeeds.
However sometimes the outage takes more than time needed
for two retries and third one also fails.

Extend retry count from 3 to 5 to improve the probablity
that the fetcher succeeds.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>