Switch vsix packing to use Nuget.config (#14209)

* Add a NuGet.config file

Author: sean-mcmanus

Build/package/jobs_package_vsix.yml

@@ -63,7 +63,7 @@ jobs:
     displayName: Install NuGet
   - task: NuGetAuthenticate@1
     displayName: Authenticate NuGet
-  - script: nuget restore $(Build.SourcesDirectory)\Build\signing\SignVsix.proj -PackagesDirectory $(Build.SourcesDirectory)\Build\signing\packages
+  - script: nuget restore $(Build.SourcesDirectory)\Build\signing\SignVsix.proj -PackagesDirectory $(Build.SourcesDirectory)\Build\signing\packages -ConfigFile $(Build.SourcesDirectory)\Build\signing\NuGet.config
     displayName: Restore MicroBuild Core
   - task: MSBuild@1
     displayName: Sign the vsix

Build/signing/NuGet.config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="Engineering" value="https://pkgs.dev.azure.com/devdiv/_packaging/MicroBuildToolset/nuget/v3/index.json" />
+  </packageSources>
+</configuration>