Fix VS MSBuild discovery

My recent change to support VS MSBuild discovery for future versions of VS accidentally broke VS
discovery altogether be leaving "MSBuild" out of the path. Oops!

src/OmniSharp.Host/MSBuild/Discovery/Providers/VisualStudioInstanceProvider.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Immutable;
+using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
 using Microsoft.Extensions.Logging;
@@ -68,7 +69,9 @@ public override ImmutableArray<MSBuildInstance> GetInstances()
                         continue;
                     }
 
-                    var toolsPath = FindMSBuildToolsPath(installPath);
+                    var msbuildPath = Path.Combine(installPath, "MSBuild");
+
+                    var toolsPath = FindMSBuildToolsPath(msbuildPath);
                     if (toolsPath != null)
                     {
                         builder.Add(