Resolve NuGet.config issues in the VMR (#16386)

Co-authored-by: Matt Thalman <mthalman@microsoft.com>

src/SourceBuild/content/NuGet.config

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    This empty NuGet.config at the root of the repo ensures that a global/default one
+    (referencing nuget.org) is not used by accident.
+    The individual repo NuGet.config files are utilized during the build.
+-->
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
+  </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
+</configuration>

src/VirtualMonoRepo/source-mappings.json

@@ -144,7 +144,9 @@
                 "src/humanizer/samples/**/*.js",
                 "src/application-insights/**/*.exe",
                 "src/application-insights/**/*.dll",
-                "src/application-insights/**/*.zip"
+                "src/application-insights/**/*.zip",
+                "src/application-insights/**/NuGet.config",
+                "src/newtonsoft-json/**/NuGet.Config"
             ]
         },
         {