Address CI failures on macOS runners

[khaeru]

Since about 2025-11-23, the "pytest" CI workflow has failed in the "warm-lfs-cache" job and the step that invokes francisbilham1/action-cached-lfs-checkout. Specifically this error message appears, e.g. here:

Error: The template is not valid. francisbilham11/action-cached-lfs-checkout/v3/action.yml (Line: 78, Col: 14): hashFiles('**/.lfs-assets-id') failed. Fail to hash files under directory '/Users/runner/work/message-ix-models/message-ix-models'

Here is the same step in a workflow run from 2025-11-22 that passed. As far as I can tell, the only difference is in this info (failing runs):

Operating System
  macOS
  15.7.2
  24G325

vs this (passing run):

Operating System
  macOS
  15.7.1
  24G231

This appears to be actions/runner-images#13341 which has got a lot of attention in 2 days and seems to be getting active attention from GitHub staff, so perhaps it will be fixed soon.

In investigating I also traced to this line blame in the third-party action we are using: https://github.com/francisbilham11/action-cached-lfs-checkout/blame/main/action.yml#L78

This uses actions/cache@v4, which is still the most recent major revision, so should still work.