This pull request updates the build process to add support for Ubuntu in addition to Windows and macOS. It introduces steps to install Mono and NuGet on Ubuntu, and updates the Makefile to use the appropriate NuGet executable depending on the operating system, improving cross-platform compatibility and reliability of builds.

Build workflow improvements:

• Added ubuntu-latest to the build matrix in .github/workflows/build.yml, enabling CI builds on Ubuntu.
• Introduced a new step to install Mono and NuGet on Ubuntu in the GitHub Actions workflow, ensuring dependencies are available for Linux builds.
• Modified the build and pack step to use the correct NuGet command on Ubuntu by setting the NUGET environment variable accordingly.

Makefile enhancements for platform compatibility:

• Updated the NUGET variable in Makefile to select the right NuGet executable based on the operating system, supporting both Windows and Linux environments.
• Replaced direct calls to nuget with $(NUGET) in prepare and nuget targets, ensuring the correct executable is used for package management across platforms. [1] [2]