Add Linux build to CI

Author: mattleibow

.github/workflows/build.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-latest, macos-latest]
+        os: [windows-latest, macos-latest, ubuntu-latest]
         configuration: [Release]
 
     steps:
@@ -34,6 +34,16 @@ jobs:
       run: |
         brew install --cask mono-mdk
 
+    - name: Install Mono - Ubuntu
+      if: matrix.os == 'ubuntu-latest'
+      run: |
+        sudo apt install 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 mono-complete mono-vbnc
+
     - name: Build and pack
       run: make prepare build nuget CONFIGURATION=${{ matrix.configuration }}