[xmlunit/xmlunit.net] c2a7b7: try building with mono on Ubuntu

[Stefan B. <no...@gi...>]

  Branch: refs/heads/main
  Home:   https://github.com/xmlunit/xmlunit.net
  Commit: c2a7b75472080b8914b2699880b07b3319c3a2f0
      https://github.com/xmlunit/xmlunit.net/commit/c2a7b75472080b8914b2699880b07b3319c3a2f0
  Author: Stefan Bodewig <bo...@ap...>
  Date:   2023-03-14 (Tue, 14 Mar 2023)

  Changed paths:
    M appveyor.yml

  Log Message:
  -----------
  try building with mono on Ubuntu