CI macos: limit number of threads for GLFW make

Using `cmake --build . --parallel` actually doesn't limit the number
of processes because it just passes the number (here none) to make,
resulting in `make -j`.

This is a possible explanations of macOS runner freezes, as eg. here:
<https://github.com/MartinPulec/UltraGrid/actions/runs/7641831068>
(last relevant message in log is `[100%] Built target joysticks`).

.github/scripts/macOS/prepare.sh

@@ -80,7 +80,7 @@ install_glfw() {(
         git fetch --depth 500 https://github.com/MartinPulec/glfw.git
         git merge FETCH_HEAD
         cmake -DBUILD_SHARED_LIBS=ON .
-        cmake --build . --parallel
+        cmake --build . -j "$(sysctl -n hw.ncpu)"
         sudo cmake --install .
 )}