only audit ETC-relevant bootnodes

[meowsbits]

https://github.com/etclabscore/core-geth/blob/master/.github/workflows/audit-bootnodes.yml https://github.com/etclabscore/core-geth/pull/589

Our bootnodes audit workflow currently audits ALL hardcoded bootnodes. Many (or all) of the unresponsive bootnodes are upstream, and merging the PR would create another difference with upstream which is not relevant to ETC. So this proposes modifying the workflow to only audit our bootnodes.