Update gcc/clang on Ubuntu CI

Only update 64bit, leave 32bit on the default gcc, gcc-10 fails:

CMake Error at /usr/local/share/cmake-3.20/Modules/CMakeTestCCompiler.cmake:66 (message):
  The C compiler

    "/usr/bin/gcc"

  is not able to compile a simple test program.

.github/workflows/main.yml

@@ -67,10 +67,12 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt install -y gcc-multilib g++-multilib cmake build-essential \
-                automake clang-6.0 g++-8 libc++-dev libogg-dev libvorbis-dev \
+                automake clang g++-10 libc++-dev libogg-dev libvorbis-dev \
                 libopenal-dev libboost-all-dev libsdl2-dev libsdl2-image-dev \
                 libfreetype6-dev libharfbuzz-dev libfribidi-dev libglib2.0-dev \
                 gtk-doc-tools rpm sshpass libraqm-dev libglew-dev libcurl4-openssl-dev libfmt-dev
+          sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 10
+          sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 10
           sudo patch /usr/include/boost/format/alt_sstream_impl.hpp mk/ubuntu/boost-allocator.patch
       - name: Install 32-bit linux dependencies
         if: ${{ matrix.os == 'ubuntu-latest' && matrix.arch == 32 }}