Fix Ubuntu CI when Microsoft breaks repositories

.github/workflows/build-ubuntu.yml

@@ -19,6 +19,7 @@ jobs:
       run: | # Fetch a new version of CMake, because the default is too old.
            wget -nv https://github.com/Kitware/CMake/releases/download/v$CMAKE_VERSION/cmake-$CMAKE_VERSION-Linux-x86_64.tar.gz \
            && tar -zxf cmake-$CMAKE_VERSION-Linux-x86_64.tar.gz \
+           && sudo rm -f /etc/apt/sources.list.d/dotnetdev.list /etc/apt/sources.list.d/microsoft-prod.list \
            && sudo apt-get update \
            && sudo apt-get install gtk+-3.0 libcurl4-gnutls-dev libpcap0.8-dev libsdl2-dev
     - name: Create build environment