source-build: don't use crossgen when building for mono runtime.

[tmds]

dotnet/installer#14792 enables building .NET from source with mono runtime instead of coreclr for 'any' architecture.
With mono runtime, crossgen2 is not available so we mustn't use it while building aspnetcore.

@dougbu ptal.

cc @MichaelSimons @crummel

[ghost]

Thanks for your PR, @tmds. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[wtgodbe]

Could we use regular crossgen?

[dougbu]

What will set $(SourceBuildUseMonoRuntime)❔ I don't see it in Arcade, at least the commit I've got locally

[tmds]

Could we use regular crossgen?

I don't know. Maybe mono doesn't support R2R assemblies? I'm just guessing.
Disabling this is what we're doing on the mono specific architectures already:

aspnetcore/src/Framework/App.Runtime/src/Microsoft.AspNetCore.App.Runtime.csproj

Lines 45 to 47 in b9dae95

	<!-- Optimize the framework using the crossgen2 tool. Crossgen2 is not currently supported on s390x or ppc64le or armv6. -->
	<CrossgenOutput Condition=" '$(TargetArchitecture)' == 's390x' OR '$(TargetArchitecture)' == 'armv6' OR '$(TargetArchitecture)' == 'ppc64le' ">false</CrossgenOutput>
	<CrossgenOutput Condition=" '$(CrossgenOutput)' == '' AND '$(Configuration)' != 'Debug' ">true</CrossgenOutput>

What will set $(SourceBuildUseMonoRuntime)grey_question I don't see it in Arcade, at least the commit I've got locally

It gets set by installer, see dotnet/installer#14792.

[dougbu]

@tmds let us (@wtgodbe and I) know if you need us to merge this for you. In addition, this repo supports the backport bot and we can automatically create similar changes in servicing branches if needed.

[tmds]

@dougbu yes, please merge this, and also backport it to 7.0.

[dougbu]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/aspnetcore/actions/runs/3430825633