Update Linux build script.

.github/workflows/build.yml

@@ -14,11 +14,10 @@ jobs:
     steps:
       - name: Check out the repository
         uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8
-      - name: Install GCC 9 and SDL2
+      - name: Install SDL2
         run: |
-          sudo add-apt-repository ppa:ubuntu-toolchain-r/test
           sudo apt-get update
-          sudo apt-get install g++-9 libsdl2-dev
+          sudo apt-get install libsdl2-dev
       - name: Build Stella
         run: |
           ./configure && make -j2 all