CI: fix code coverage errors
It fails too many times with multithread. Hopefully it will not fail with a single thread.
The meson build is only allowed to fail when some meson support will be added later. It should not fail 90% of the time.
The extra option suggested by the error log seems to do the trick, even when using all threads possible.
We need to check how much longer it takes to generate the files compared to (successful) 4 threads.
IMO 35s more is OK. This is still a short job compared to the others.