Create Microsoft.DotNet.Compatibility NuGet during source-build

[eerhardt]

The Microsoft.DotNet.Compatibility NuGet package is a new package created in 6.0. The dotnet/runtime repo takes a dependency on this package during its build:

https://github.com/dotnet/runtime/blob/825b26409226be4a29c7dcbe5aab97b979e3dac0/eng/packaging.targets#L40-L42

However, this package isn't getting created during source-build. As far as I can tell, this is because we are using a .slnf file that doesn't include it:

https://github.com/dotnet/sdk/blob/599fde6a0276a1e9d8eefea4ee4e4a4f9518bfd8/source-build.slnf#L16

We should add the Microsoft.DotNet.Compatibility project to source-build, so the NuGet package gets created during source-build. That way the dotnet/runtime repo will pick up the source-built package and not use a pre-built version.

[MichaelSimons]

This is blocked on these runtime changes flowing into installer - dotnet/runtime#60306. After they flow, we will need to update previous source-built and the prebuilt should be resolved.

[MichaelSimons]

Added with dotnet/sdk#21978.