## Summary

- The combined publish workflows (`release-70`, `release-22`) call
multiple sub-workflows that each invoke `check-synchronization.yml`.
That reusable workflow has a concurrency group (`${{ github.repository
}}-${{ github.workflow }}-${{ github.ref }}`) that evaluates to the
**same value** for all callers within a single run, causing GitHub to
randomly cancel competing jobs.
- PR #11631 attempted to fix this with `github.event_name ==
'workflow_dispatch'` guards, but `github.event_name` propagates the
original trigger through `workflow_call` chains, so the guard doesn't
reliably skip the check.
- This PR replaces that guard with an explicit `skip_sync_check` boolean
input. Combined workflows pass `skip_sync_check: true` since they
already run the check at the top level. Standalone dispatches default to
`false`, preserving existing behavior.
- Also removes the redundant `check-synchronization` job from
`release-reusable-publish-packages.yml` (only ever called from workflows
that already perform the check).

Evidence: [4 consecutive failed
attempts](https://github.com/paritytech-release/polkadot-sdk/actions/runs/23787944175)
with different random jobs cancelled each time.

## Issue

- paritytech/release-engineering#291

---------

Co-authored-by: Egor_P <egor@parity.io>
(cherry picked from commit 9f6182f)