ci: clean self-hosted runner workspace after each job

Self-hosted runners don't auto-clean their workspace between jobs, so
PR fork content + cloned vyos-1x source + downloaded reference DB +
the App-token-bearing pip install cache all linger on the runner host
until the next job overwrites them. That's a leak surface for both
secret material and disk space.

Adds atos-actions/clean-self-hosted-runner@v1.4.34 (SHA-pinned) as the
LAST step in both prepare and validate jobs, gated with `if: always()`
so it runs after success, failure, or cancellation.

The action is from atos-actions, a verified GitHub Marketplace partner.
Composite action — no Node 20/24 deprecation. Logic is auditable
(rm -rf ./* ./.[!.]*). Has a DISABLE_RUNNER_CLEANUP env var kill-switch
for debugging if a job needs to leave residual state for inspection.

🤖 Generated by [robots](https://vyos.io)

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