fix(publish-helm-chart): skip plugin verification for helm v4 compat

[sydorovdmytro]

Summary

• Helm v4.1.4 (installed by default since chore(deps): update dependency helm/helm to v4 #106) verifies plugin sources on helm plugin install. The chartmuseum/helm-push plugin does not ship a verification manifest, so the install fails:

  Error: plugin source does not support verification. Use --verify=false to skip verification
  

• Passing --verify=false fixes the install. The flag is also accepted on v3 (as a no-op), so callers pinning helm-version: v3.x are unaffected.

Impact

Broke the nightly head-chart publish in loft-enterprise after loft-sh/loft-enterprise#6673 adopted this reusable workflow. Any other caller using publish-helm-chart/v1 is affected the same way.

Follow-up

After merge, move the publish-helm-chart/v1 tag to this commit:

git tag -f publish-helm-chart/v1 <merged-sha>
git push origin publish-helm-chart/v1 --force

Test plan

• make lint passes locally (actionlint + zizmor clean)
• make test-publish-helm-chart runs (bats unchanged — the shell script wasn't touched)
• Verified end-to-end by the next head-chart push in loft-enterprise once the publish-helm-chart/v1 tag is moved