CI: fix macOS project configuration

Author: pinam45

.github/workflows/macos.yml

@@ -58,6 +58,7 @@ jobs:
         ${{steps.cmake.outputs.cmake_binary}}
         -S "${{github.workspace}}/dynamic_bitset"
         -B "${{github.workspace}}/dynamic_bitset/build"
+        -G Xcode
         -D CMAKE_CONFIGURATION_TYPES=${{matrix.build_type}}
         -D CMAKE_BUILD_TYPE=${{matrix.build_type}}