Allow optional caching in failed builds

What does this MR do?

Introduces cache:when to the ci config.

cache:when defines when to save the cache, based on the status of the job. You can set cache:when to:

• on_success - save the cache only when the job succeeds. This is the default.
• on_failure - save the cache only when the job fails.
• always - save the cache regardless of the job status.

For example, to store a cache whether or not the job fails or succeeds:

rspec:
  script: rspec
  cache:
    paths:
      - rspec/
    when: 'always'

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Part of #18969 (closed)