[xabuild] Probe DOTNET_ROOT for .NET sdks

[jonpryor]

Context: #4467
Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=3590265&view=logs&j=96fd57f5-f69e-53c7-3d47-f67e6cf9b93e&t=7b19ceb3-907a-5a0d-b8ed-87e3f7335a58

When trying to integration test dotnet/java-interop#614, the
make all-tests job fails:

/usr/local/share/dotnet/sdk/2.2.402/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(137,5):
error NETSDK1045: The current .NET SDK does not support targeting .NET Core 3.1.  Either target .NET Core 2.2 or lower, or use a version of the .NET SDK that supports .NET Core 3.1.
[/Users/builder/azdo/_work/2/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.csproj]

Particularly odd about this is that we have an explicit job to install
.NET Core 3.1.100, so why is .NET Core 2.2.402 being used at all?!

Turns Out™ that while .NET Core 3.1.100 was installed, it was
installed into a different prefix; from the
install .NET Core 3.1.100 job:

##[debug]set DOTNET_ROOT=/Users/builder/azdo/_work/_tool/dotnet

and from the make jenkins job output:

DOTNET_ROOT = /Users/builder/azdo/_work/_tool/dotnet

Thus, the install .NET Core 3.1.100 job is installing .NET Core into
$HOME/azdo/_work/_tool/dotnet, while make all-tests is only
looking for .NET Core SDKs within /usr/local/share/dotnet, which
does not contain .NET Core 3.1 at all.

Where does the make all-tests path come from? It comes from
tools/xabuild, which sets the MSBuildSDKsPath environment
variable, and xabuild did not check $DOTNET_ROOT.

Update xabuild so that $DOTNET_ROOT is also probed for .NET Core
installation directories, and use the latest .NET Core SDK path
found between $DOTNET_ROOT and /usr/local/share/dotnet.