Update dotnet.yml

Author: mrdav30

.github/workflows/dotnet.yml

@@ -26,8 +26,12 @@ jobs:
 
     - name: Install Mono
       run: |
-        sudo apt update
-        sudo apt install -y mono-complete
+        apt update
+        apt install -y ca-certificates gnupg
+        gpg --homedir /tmp --no-default-keyring --keyring /usr/share/keyrings/mono-official-archive-keyring.gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+        echo "deb [signed-by=/usr/share/keyrings/mono-official-archive-keyring.gpg] https://download.mono-project.com/repo/ubuntu stable-focal main" | tee /etc/apt/sources.list.d/mono-official-stable.list
+        apt update
+        apt install -y mono-complete
 
     - name: Cache NuGet packages
       uses: actions/cache@v3