Bump unix toolset version to VS 2017 RTW compiler version (#18404)

build/MSBuildToolset/project.json

@@ -11,9 +11,9 @@
     "Microsoft.Build": "15.1.0-preview-000458-02",
     "Microsoft.Build.Runtime": "15.1.0-preview-000458-02",
     "Microsoft.Build.Tasks.Core": "15.1.0-preview-000458-02",
-    "Microsoft.Net.Compilers.netcore": "2.0.0-rc2-61102-09",
+    "Microsoft.Net.Compilers.netcore": "2.0.0-rc4-61325-08",
     "Microsoft.NuGet.Build.Tasks": "0.1.0",
-    "Microsoft.CodeAnalysis.Build.Tasks": "2.0.0-rc2-61102-09",
+    "Microsoft.CodeAnalysis.Build.Tasks": "2.0.0-rc4-61325-08",
     "Microsoft.NETCore.TestHost": "1.1.0",
     "Microsoft.NETCore.App": "1.1.0",
     "Microsoft.Portable.Targets": "0.1.2-dev",