Revert "Use ubuntu 1804 arm64 queue outerloop (dotnet/coreclr#26984)" (…

…dotnet/coreclr#27262)

This reverts commit dotnet/coreclr@54d536f.

Commit migrated from dotnet/coreclr@7c9ea85

eng/platform-matrix-managed-test-build.yml

@@ -72,9 +72,8 @@ jobs:
         image: ubuntu-16.04-cross-arm64-cfdd435-20190923200213
         registry: mcr
       helixQueues:
-      # Standardize on one set of hardware, when we can handle load in one queue.
       - ${{ if eq(variables['System.TeamProject'], 'public') }}:
-        - (Ubuntu.1804.Arm64.Open)Ubuntu.1804.Armarch[email protected]/dotnet-buildtools/prereqs:ubuntu-18.04-helix-arm64v8-a45aeeb-20190620155855
+        - (Ubuntu.1804.Arm64.Open)Ubuntu.1604.Arm64.Docker[email protected]/dotnet-buildtools/prereqs:ubuntu-18.04-helix-arm64v8-a45aeeb-20190620155855
       - ${{ if and(eq(variables['System.TeamProject'], 'public'), notIn(parameters.helixQueueGroup, 'pr', 'ci', 'corefx')) }}:
         - (Debian.9.Arm64.Open)[email protected]/dotnet-buildtools/prereqs:debian-9-helix-arm64v8-74c9941-20190620155840
       - ${{ if eq(variables['System.TeamProject'], 'internal') }}: