Add option to actions/cache to not save conditionally

[EnricoMi]

Description

Add an option to control the save post job.

Motivation and Context

With such an option, the save post job can be triggered conditionally like

- name: Cache
  uses: actions/cache@v4
  with:
    path: .cache
    key: ${{ runner.os }}-cache
    save: ${{  github.ref == 'refs/heads/master' }}

With that, there is no need to split the actions/cache step into a restore and save step which makes the workflow cluttered.

How Has This Been Tested?

Visual inspection of the test-dont-save jobs.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (add or update README or docs)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[EnricoMi]

The test-dont-save jobs shows that this is not sufficient to skip the post job if save: false. Pointers to the right place to add the if (doSave) appreciated.