added more recent GCC version to CI

.github/workflows/cmake.yml

@@ -18,7 +18,7 @@ jobs:
     - uses: actions/checkout@v2
     
     - name: Install dependencies
-      run: sudo apt install libfftw3-dev libglfw3-dev libglew-dev libvolk1-dev libsoapysdr-dev libairspyhf-dev libiio-dev libad9361-dev portaudio19-dev
+      run: sudo apt install gcc-8 g++-8 libfftw3-dev libglfw3-dev libglew-dev libvolk1-dev libsoapysdr-dev libairspyhf-dev libiio-dev libad9361-dev portaudio19-dev
 
     - name: Create Build Environment
       # Some projects don't allow in-source building, so create a separate build directory