Fixed bad directory for MacOS CI

.github/workflows/build_all.yml

@@ -65,6 +65,7 @@ jobs:
           run: brew install fftw glew glfw volk airspy rt-audio hackrf rtl-sdr
 
         - name: Prepare CMake
+          working-directory: ${{runner.workspace}}/build
           run: cmake $GITHUB_WORKSPACE -DOPT_BUILD_AIRSPYHF_SOURCE=OFF -DOPT_BUILD_PLUTOSDR_SOURCE=OFF -DOPT_BUILD_SOAPY_SOURCE=OFF
 
         - name: Build