fix: port reusable network-teardown action to v5-next

[PhilWindle]

What

Cherry-picks ef43cb7fa20 ("ci: reusable network-teardown action that runs on GitHub runners") from next onto the v5 line.

This is a clean port — .github/actions/network-teardown/action.yml is byte-identical to next, and the three workflow files (nightly-bench-10tps.yml, nightly-spartan-bench.yml, test-network-scenarios.yml) get the same teardown-refactor hunks applied on top of their v5-specific config.

Why

The nightly Spartan Benchmarks run on the private repo's default branch (next), whose cleanup-* jobs were refactored on 2026-06-17 to call the local composite action:

- name: Checkout
  with:
    ref: ${{ needs.select-image.outputs.source_ref }}   # the nightly TAG
- name: Cleanup network resources
  uses: ./.github/actions/network-teardown

The checkout pins source_ref (the nightly tag). When the deployed tag is a v5 nightly tag, that tree did not contain .github/actions/network-teardown (the action was only ever on the next line), so cleanup failed with:

Can't find 'action.yml' … under '.github/actions/network-teardown'.
Did you forget to run actions/checkout before running your local action?

This left network resources orphaned on every failure (observed in nightly run #95). Porting the action to v5-next means future v5 nightly tags include it, so the cleanup checkout resolves the action correctly.

weekly-proving-bench.yml and ci3.yml keep the older ci3.sh network-teardown form — matching next exactly (the original commit did not touch them).

Routing

Targets v5-next directly (not via merge-train/spartan-v5) since the commit already exists on next; routing through the merge train would attempt a redundant port back to private next. This PR syncs to private v5-next.