Add Roslyn 4.4 variant build step to CI workflow

Co-authored-by: atifaziz <20511+atifaziz@users.noreply.github.com>

Author: Copilot

.github/workflows/ci.yml

@@ -50,6 +50,9 @@ jobs:
     - name: Build
       run: dotnet build --configuration ${{ env.CONFIGURATION }}
 
+    - name: Build Roslyn 4.4 variant
+      run: dotnet build src/DocoptNet/DocoptNet.csproj -f netstandard2.0 -p:RoslynVersion=4.4 --configuration ${{ env.CONFIGURATION }}
+
     - name: Pack (Windows only)
       if: runner.os == 'Windows'
       shell: pwsh