Ensure we are not using MyGet for releases

NuGet.release.config

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+  <disabledPackageSources />
+</configuration>

appveyor.yml

@@ -15,7 +15,10 @@ install:
   - ps: $env:DOTNET_SKIP_FIRST_TIME_EXPERIENCE = true
   - ps: $env:NUGET_XMLDOC_MODE = "skip"
   - ps: $env:DOTNET_CLI_TELEMETRY_OPTOUT = 1
+  - ps: $IsReleaseBranch = ($env:APPVEYOR_REPO_BRANCH -eq "master" -Or $env:APPVEYOR_REPO_BRANCH -eq "rel/3.0-beta")
 build_script:
+  # Ensure we are not using the myget feed for dependencies
+  - ps: if ($IsReleaseBranch -or $IsDevBranch) { copy NuGet.release.config NuGet.config }
   - dotnet --version
   - dotnet pack -c Release
 test_script: