Add Mono installation step to pipeline before NuGet pack (#462)

Added a script step to install Mono (mono-complete) on the build
agent prior to running the NuGet pack task. This ensures that
Mono is available for .NET/Mono-based tooling or packaging,
improving compatibility on Linux agents.

Author: pim-simons

build/psgallery-release.yml

@@ -186,6 +186,10 @@ stages:
             displayName: 'Generate .nuspec file for each PowerShell module'
             env:
              SCRIPT_PATH: '$(Build.SourcesDirectory)\build\tools\psd1-to-nuspec.ps1'
+          - script: |
+              sudo apt update
+              sudo apt install -y mono-complete
+            displayName: Install Mono
           - task: NuGetCommand@2
             displayName: 'NuGet Pack'
             inputs: