GH CI: do not run on MacOS since it's broken

[smelc]

Changelog

- description: |
    GH CI: do not run on MacOS since it's broken
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

As recently updated PRs on API show, the MacOS CI is broken right now, see for example

on #691 and #692

This failure was raised on Slack but didn't gain traction: https://input-output-rnd.slack.com/archives/CG1FBSDMM/p1732543089865619

So this PR turns off the GitHub action build on MacOS for now.

How to trust this PR

It's less checks. Don't trust it. But Hydra still runs on Darwin, so we should be covered.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff

[smelc]

Closing: superseded by #694