Add SKIP check to workflows removed in v15

[mattlord]

Description

These cluster_initial_sharding_multi.yml and legacy_local_example.yml workflows did not exist on main — they were removed in #10278 — so they were not updated when #10768 was backported.

This means that we'll need to backport this PR to release-13.0 and release-12.0 as well.

Related Issue(s)

• Follow-up to: Skip CI workflows on push for pull requests #10768
• Follow-up to: Delete all legacy sharding related code #10278
• Fixes test failures in: [release-14.0] fix: return when instructions are nil in checkThatPlanIsValid (#11070) #11077

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests are not required
• Documentation is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a new flag is being introduced, review whether it is really needed. The flag names should be clear and intuitive (as far as possible), and the flag's help should be descriptive. Additionally, flag names should use dashes (-) as word separators rather than underscores (_).
• If a workflow is added or modified, each items in Jobs should be named in order to mark it as required. If the workflow should be required, the GitHub Admin should be notified.

Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should either include a link to an issue that describes the bug OR an actual description of the bug and how to reproduce, along with a description of the fix.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.

[frouioui]

We must add the if conditions that skip the workflow if needed. I am going to commit this @mattlord

[vitess-bot]

I was unable to backport this Pull Request to the following branches: release-13.0, release-12.0.

[frouioui]

release-13.0 already contains this change, and release-12.0 does not have the skip workflow feature yet.

No backports needed.