CI: Linux/MacOS Verification jobs exhaust organisation quota of runners

[CodeGat]

Background

ACCESS-NRI/ has a maximum job concurrency limit of 20 across all repositories in the organisation. The https://github.com/ACCESS-NRI/MOM6/blob/2026.05/.github/workflows/verify-linux.yml and https://github.com/ACCESS-NRI/MOM6/blob/2026.05/.github/workflows/verify-macos.yml have many individual jobs that lead to a single commit in MOM6 exhausting the runner pool for the organisation.

Implementation Details

• Consider reworking the list of individual jobs into a matrix of jobs, with jobs.*.strategy.max-parallel set to something lower, like 10

[dougiesquire]

Thanks @CodeGat. These are inherited from upstream. We can modify the implementation but that will require carrying the mods in our fork into the future