Test building source-build with a mono-based runtime too

[omajid]

In source-build, we mostly build/test/integrate with the coreclr-based runtime. We assume that coreclr is the only available runtime for .NET as well as source-build.

However, mono, not coreclr, is used on certain architectures that we would really like source-build to support, such as s390x.

Due to a lack of building/testing the mono based runtime, it's possible that we will have errors/regressions that we miss in source-build. For example, consider prebuilt reduction. We may remove or conditionalize a package based on DotnetBuildFromSource in coreclr, but it could still be an unconditionalized required dependency (possibly a prebuilt) when doing a mono-based build.

It should be possible to run this sort of test on x64 as well, assuming we can find a way to configure the runtime repository to build with mono instead of coreclr on x64.

Preferably, this should be all automated.

cc @uweigand @crummel