Turn off coverage reporting in MacOS

Something is strange about the coverage reported by MacOS builds. Perhaps llvm-cov? Turning off whilst we investigate.

Relates to #279 and "solves" it by ignoring the problem.

.github/workflows/ci.yml

@@ -143,7 +143,7 @@ jobs:
           lcov --list coverage.info
 
       - name: Upload coverage reports to Codecov
-        if: matrix.build_testing == 'ON'
+        if: matrix.build_testing == 'ON' && ${{ contains(matrix.os, 'ubuntu') }}
         uses: codecov/codecov-action@v3
 
       - name: Tar the build result to maintain permissions