cmake: generate for Ninja when the build phase is Ninja

This is overridable by providing a custom build phase or setting
dontUseNinjaBuild = true.

pkgs/development/tools/build-managers/cmake/setup-hook.sh

···
     # And build always Release, to ensure optimisation flags
     cmakeFlags="-DCMAKE_BUILD_TYPE=${cmakeBuildType:-Release} -DCMAKE_SKIP_BUILD_RPATH=ON $cmakeFlags"
 
+    if [ "$buildPhase" = ninjaBuildPhase ]; then
+        cmakeFlags="-GNinja $cmakeFlags"
+    fi
+
     echo "cmake flags: $cmakeFlags ${cmakeFlagsArray[@]}"
 
     cmake ${cmakeDir:-.} $cmakeFlags "${cmakeFlagsArray[@]}"