grep -q exits as soon as it matches which can cause broken pipes if echo is not finished with the output.

It should hopefully fix the issues with calculating the matrix in Ubuntu 24 (no idea why the Ubuntu 24 runners are more sensitive...).

@holly-cummins this should be the actual fix for the matrix calculation issue in Ubuntu 24. I'm not exactly sure why we got lucky when testing my previous fix. My intuition is that maybe the Ubuntu 24 runner are a bit faster and can cause these issues sporadically (but when I tested last time, it was always reproducible so unsure...).