[release/3.1] Use Microsoft.NETCore.App.Internal for runtime version (#17627)

Author: wtgodbe

global.json

@@ -6,10 +6,10 @@
     "dotnet": "3.1.100-preview1-014400",
     "runtimes": {
       "dotnet/x64": [
-        "$(MicrosoftNETCoreAppRuntimeVersion)"
+        "$(MicrosoftNETCoreAppInternalPackageVersion)"
       ],
       "dotnet/x86": [
-        "$(MicrosoftNETCoreAppRuntimeVersion)"
+        "$(MicrosoftNETCoreAppInternalPackageVersion)"
       ]
     },
     "Git": "2.22.0",