chore(ci): public nightlies skip scenario tests and the next tag

[AztecBot]

What

Two changes scoped to the public repo (AztecProtocol/aztec-packages) nightly flow, plus a follow-up tightening of the scenario-test trigger. Private tagging is unchanged.

1. Network scenario tests run only on the private v5-next nightly

ci3.yml's ci-network-scenario job fired on any current nightly tag in both repos. Private produces both a next (v6) and a v5-next (v5) nightly tag, so simply gating to the private repo still ran scenarios against the v6 nightly. The nightly-triggered path is now gated to private repo + a v5. nightly tag:

(
  needs.validate-nightly-tag.outputs.is_current == 'true'
  && github.repository == 'AztecProtocol/aztec-packages-private'
  && startsWith(github.ref_name, 'v5.')
)
|| contains(github.event.pull_request.labels.*.name, 'ci-network-scenario')

v5-next is at 5.x.x (tag v5.x.x-nightly.*) and next is at 6.x.x (tag v6.x.x-nightly.*), so startsWith(github.ref_name, 'v5.') selects the v5-next nightly only. The manual PR-label path (ci-network-scenario) is preserved for ad-hoc dev runs.

2. Stop tagging next with a nightly tag in public

nightly-release-tag.yml's matrix tagged [next, v5-next] in both repos. The branch list is now repo-dependent: private keeps [next, v5-next], public tags only v5-next (and v4-next via its existing dedicated job). Net result: public tags v4-next + v5-next only, private is untouched.

Why

Nightly network scenario tests should run only against the private v5-next nightly, and public should not produce a next nightly tag.