chore: install mono in workflow

Author: stratosblue

.github/workflows/PublishNugetPackage.yml

@@ -22,6 +22,15 @@ jobs:
           8.x
           9.x
           10.x
+    - name: Install Mono
+      run: |
+        sudo apt install ca-certificates gnupg
+        sudo 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" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
+        sudo apt update
+        sudo apt install -y mono-complete
+        mono --version
+
     - name: build
       run: dotnet build -c Release
     - name: test