Fix PDK Update workflow: add workflows: write permission

[Copilot]

PDK manages .github/workflows/nightly.yml (via .sync.yml unmanaged: false), so pdk update modifies it. GitHub rejects pushes containing workflow file changes unless the token has workflows: write — causing the weekly PDK Update job to fail at the push step.

Summary

Add workflows: write to the permissions block in .github/workflows/pdk_update.yml:

permissions:
  contents: write
  pull-requests: write
  workflows: write   # required: pdk update modifies .github/workflows/nightly.yml

Additional Context

• Root cause and the steps to reproduce. (If applicable)
  • pdk update --force modifies nightly.yml → peter-evans/create-pull-request tries to push a branch with that change → GitHub rejects with: refusing to allow a GitHub App to create or update workflow ... without workflows permission
• Thought process behind the implementation.
  • Minimal permission addition; no logic changes needed.

Related Issues (if any)

N/A

Checklist

• 🟢 Spec tests.
• 🟢 Acceptance tests.
• Manually verified. (For example puppet apply)