Install mono manually on ubuntu-latest

Author: dnperfors

.github/workflows/ci.yml

@@ -55,6 +55,17 @@ jobs:
       uses: actions/checkout@v5
     - name: Fetch all history for all tags and branches
       run: git fetch --prune --unshallow
+    - name: Install Mono (Ubuntu only)
+      if: matrix.os == 'ubuntu-latest'
+      run: |
+        sudo apt install -y 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
     - name: Setup .NET versions
       uses: actions/setup-dotnet@v5
       with: