.gitlab-ci.yml: use "clone" strategy for macOS jobs

Description

• So permissions are correctly reset on any failed jobs; see: https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/3726
• With this fix, after_script no longer needed, and interruptible should be safe
• Also rename *macosx* jobs to *macos*
• Ref #661

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

cc @adam-mercer