Fix PR status checks (#18895)
Fixes #18869
### Summary of the issue:
Status checks are not properly enforced, meaning PRs with failing checks
are allowed to be merged.
### Description of user facing changes:
None
### Description of developer facing changes:
CI checks must pass
### Description of development approach:
Use [alls-green](https://github.com/marketplace/actions/alls-green).
### Testing strategy:
Create test failures and ensure they disable merging.
* Intentionally introduced a unit test failure in [commit
`aaa4146d`](https://github.com/nvaccess/nvda/pull/18895/commits/aaa4146d0971cd639d731635a2135821a6ce969c)
([CI run](https://github.com/nvaccess/nvda/actions/runs/17628492155))
* Intentionally introduced a system test failure in [commit
`6cb4d8ad`](https://github.com/nvaccess/nvda/pull/18895/commits/6cb4d8adc6eb72bc285e00fba9248b5a6809f716)
([CI run](https://github.com/nvaccess/nvda/actions/runs/17629261960))
I did not check po, pot, lint, types etc, however I believe that the
above tests are sufficient to prove that this approach works.
### Known issues with pull request:
None
