ci: improve helm repo update

[cjorge-graphops]

Currently helm repo index generation process needs to download all releases and in particular for the canary variant the time it takes has been building up over time. During this lengthy process, sometimes a request fails which fails the action.

There are two improvements we could quickly make:

• Only keep canary versions if they are the latest, or not older than X time (1 month?)
• Retry the API requests