Github Actions: disable mac builds

[smatting]

Disabling mac builds (which are 10x more expensive) because nobody in the team needs these caches atm.

We enabled these in #2348

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.