Description

Add support for not saving the cache.

Fixes #350
Fixes #334
Fixes #92

Similar to both #489 and #964, but here it's implemented as an input to the workflow instead of an environment variable.

Motivation and Context

I needed the behavior for this PR: github/codeql#11362.

In github/codeql we have some code that gets faster by using a cache.
But that code can take some shortcuts if it doesn't have to save a new cache.
So on PRs we essentially run in a "read-only" mode, where we don't expect any cache to be populated.
And we make sure that the cache is populated on pushes to main.

Our current workaround is to use a key based on the current commit SHA, which no other run will hit.
But that is currently filling up the cache space in the repo: https://github.com/github/codeql/actions/caches

How Has This Been Tested?

• I added a test-case.
• It's been concretely tested on a workflow here: CI: use read-only-cache when running on a PR github/codeql#11362

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'll do that if the PR looks OK).
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.