ci: use shared wiz-iac-scan reusable workflow

[rguichard]

Replaces the local wiz-iac-scan.yml with a thin caller of the shared reusable morpho-org/github-actions/.github/workflows/wiz-iac-scan.yml@v1.

Iso behavior preserved: trigger pull_request on helm/**, scan-path helm/, runner arc-morpho-infra-dev (reusable default), no policies. Wiz CLI version+SHA256 and the fork-PR guard now live centrally.

Part of PLA-1787
Implements PLA-1788

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[rguichard]

Closing: erpc's inline wiz-iac-scan.yml (GitHub Actions) is vestigial — every historical run is cancelled (never a green check), and erpc lacks the org-scoped WIZ_CLIENT_ID/SECRET secrets, so calling the shared reusable (which declares them required) fails at startup. erpc's real IaC/secret scanning is the native Wiz app integration (the green Wiz IaC Scanner / Wiz Secret Scanner checks), which is unaffected. Recommendation: delete the redundant inline workflow in a separate PR rather than migrate it. Tracked under PLA-1788.