Fix the git describe in update-helm-repo to be Helm-centered

[yorugac]

Current update-helm-repo workflow decides whether to make a Helm chart release based on diff from the latest tag. But the latest tag may not necessarily be Helm-related which can lead to a wrong decision.

Background. I'm currently re-working release procedure in k6-operator so that both the main app and Helm chart are published simultaneously, with automations. And in that case, update-helm-repo may see the latest tag from the operator release (like v1.2.3) on HEAD and decide to skip the Helm release. To avoid that, it should instead check the latest Helm release tag which has a configured prefix.

I was diagnosing this case with LLM's help, and this PR is meant to prevent a likely failure on the next release with my new release procedure. The fix adds stricter guarantees:

• users of the workflow who configure the prefix will have this more precise lookup of the previous Helm tag;
• users of the workflow who don't configure the prefix aren't impacted.

[yorugac]

Zizmor has been failing even before this change so its unrelated:

[yorugac]

I've just tried the new release scheme (to have an app tag and Helm tag on the same commit): confirming this is a real issue.
The first publish Helm workflow was skipped because it looked up the wrong, non-Helm tag:

https://github.com/grafana/k6-operator/actions/runs/28027177327

To bypass this, I had to delete the non-Helm tag manually (later re-create manually), to force Helm workflow to publish the chart properly in another run.