[xabuild] Probe DOTNET_ROOT for .NET sdks

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`.

Author: jonpryor

tools/xabuild/XABuildPaths.cs

@@ -142,14 +142,17 @@ public XABuildPaths ()
 			string programFiles       = Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86);
 			string prefix             = Path.Combine (XamarinAndroidBuildOutput, "lib", "xamarin.android");
 
+			string DOTNET_ROOT        = Environment.GetEnvironmentVariable ("DOTNET_ROOT");
+			string dotnetRootSdkDir   = DOTNET_ROOT == null ? null : Path.Combine (DOTNET_ROOT, "sdk");
+
 			if (IsWindows) {
 				var instance = MSBuildLocator.QueryLatest ();
 				VsInstallRoot = instance.VisualStudioRootPath;
 
 				MSBuildPath              = Path.Combine (VsInstallRoot, "MSBuild");
 				MSBuildBin               = Path.GetDirectoryName (instance.MSBuildPath);
 				MSBuildConfig            = Path.Combine (MSBuildBin, "MSBuild.exe.config");
-				DotNetSdkPath            = FindLatestDotNetSdk (Path.Combine (Environment.GetEnvironmentVariable ("ProgramW6432"), "dotnet", "sdk"));
+				DotNetSdkPath            = FindLatestDotNetSdk (Path.Combine (Environment.GetEnvironmentVariable ("ProgramW6432"), "dotnet", "sdk"), dotnetRootSdkDir);
 				MSBuildSdksPath          = DotNetSdkPath ?? Path.Combine (MSBuildPath, "Sdks");
 				SystemFrameworks         = Path.Combine (programFiles, "Reference Assemblies", "Microsoft", "Framework");
 				string msbuildDir        = Path.GetDirectoryName (MSBuildBin);
@@ -184,7 +187,7 @@ public XABuildPaths ()
 					throw new InvalidOperationException ("Unable to locate MSBuild binaries directory");
 
 				MSBuildConfig            = Path.Combine (MSBuildBin, "MSBuild.dll.config");
-				DotNetSdkPath            = FindLatestDotNetSdk ("/usr/local/share/dotnet/sdk");
+				DotNetSdkPath            = FindLatestDotNetSdk ("/usr/local/share/dotnet/sdk", dotnetRootSdkDir);
 				MSBuildSdksPath          = DotNetSdkPath ?? Path.Combine (MSBuildBin, "Sdks");
 				SystemFrameworks         = Path.Combine (mono, "xbuild-frameworks");
 				MonoSystemFrameworkRoot  = mono;
@@ -291,11 +294,14 @@ string RunPathsTargets (string pathsTargets, string target)
 			}
 		}
 
-		string FindLatestDotNetSdk (string dotNetPath)
+		string FindLatestDotNetSdk (params string[] dotNetPaths)
 		{
-			if (Directory.Exists (dotNetPath)) {
-				Version latest = new Version (0,0);
-				string Sdk = null;
+			Version latest  = new Version (0,0);
+			string Sdk      = null;
+
+			foreach (var dotNetPath in dotNetPaths) {
+				if (!Directory.Exists (dotNetPath))
+					continue;
 				foreach (var dir in Directory.EnumerateDirectories (dotNetPath)) {
 					var version = GetVersionFromDirectory (dir);
 					var sdksDir = Path.Combine (dir, "Sdks");
@@ -308,9 +314,8 @@ string FindLatestDotNetSdk (string dotNetPath)
 						}
 					}
 				}
-				return Sdk;
 			}
-			return null;
+			return Sdk;
 		}
 
 		static Version GetVersionFromDirectory(string dir)