feat(ci): run next-net, benches and scenarios from public v6 nightlies

[PhilWindle]

What

Restores a public v6 nightly and repoints the nightly consumers (next-net deploy, spartan benches, ci3 scenario tests) at it. Also fixes the bench-inclusion-sweep validation error.

Why

After next bumped to 6.0.0 and v5-next split off (release-machinery port, 7ac407c909), the public repo:

• narrowed nightly tagging to v5-next only → no public 6.0.0-nightly image was ever built (aztecprotocol/aztec:6.0.0-nightly.* = 404), and
• gated every consumer (next-net, spartan benches, ci3 ci-network-scenario) to aztec-packages-private + the internal registry.

Net effect: from the public side, nightly deploys/tests/benches looked stopped. The public repo still published v5.0.0-nightly daily, but nothing public produced or consumed a v6 nightly.

Changes

• nightly-release-tag.yml — public repo now tags next (v6) as well as v5-next. The tag push runs ci-release, which publishes aztecprotocol/aztec:6.0.0-nightly (the public release path has no v5 gate).
• deploy-next-net.yml — public-gated; derives the version from next's manifest; wait-for-ci3 blocks on the release build; deploys the public image (dropped use_internal_docker_registry).
• nightly-spartan-bench.yml — private→public gates; checks out next; public registry; added a wait-for-ci3 gate so the three deploy tracks don't race the release build (replaces the point-in-time image-existence check).
• ci3.yml (ci-network-scenario) — gate flipped to aztec-packages and v6., so nightly scenario tests run in the public repo on the v6 tag (runs inside the tag's ci3 run, gated on the release job).
• delete nightly-release-tag-v5-next.yml — redundant now that the matrix tags v5-next in the public repo.
• nightly-bench-inclusion-sweep.yml — pass skip_notify_on_failure: true (the renamed deploy-network input) instead of the removed notify_on_failure, fixing the "Invalid input" workflow error.

Notes / side effects

• v6 nightly now publishes publicly to DockerHub and npmjs/crates.io (same as v5 today) — inherent to publishing a public v6 nightly.
• weekly-proving-bench.yml needs no change — it already targets next+public, so it starts working once the v6 image publishes.
• Scenario tests now run on v6 only; v5 is still deployed via deploy-staging-* but no longer scenario-tested nightly.
• Worth confirming the public repo's GCP_SA_KEY can deploy next-net to CLUSTER=aztec-gke-private (staging-public deploys fine from public creds).