Update tag references to dotnet-buildtools/prereqs (dotnet#7608)

Author: mthalman

azure-pipelines.yml

@@ -20,7 +20,7 @@ variables:
 resources:
   containers:
   - container: LinuxContainer
-    image: microsoft/dotnet-buildtools-prereqs:ubuntu-14.04-cross-0cd4667-20170319080304
+    image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-14.04-cross-0cd4667-20170319080304
 
 stages:
 - stage: build