Add Mono installation to CI workflow

- Install mono-devel package before running tests
- Follows official Mono installation instructions for Ubuntu
- Required for running nuget.exe via Mono on GitHub Actions
- Ensures tests can execute properly in CI environment

Author: gamma

.github/workflows/ci.yml

@@ -42,6 +42,16 @@ jobs:
             ${{ runner.os }}-gradle-${{ matrix.gradle-version }}-
             ${{ runner.os }}-gradle-
       
+      - name: Install Mono
+        run: |
+          sudo apt install -y ca-certificates gnupg
+          sudo gpg --homedir /tmp --no-default-keyring --keyring gnupg-ring:/usr/share/keyrings/mono-official-archive-keyring.gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+          sudo chmod +r /usr/share/keyrings/mono-official-archive-keyring.gpg
+          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-devel
+          mono --version
+      
       - name: Run tests with Gradle ${{ matrix.gradle-version }}
         run: gradle clean test --no-daemon