Move to Core-Eng build agents

- should resolve problems w/ disk space in "Tests: Helix ARM64 matrix" job

Author: dougbu

.azure/pipelines/helix-matrix.yml

@@ -64,6 +64,7 @@ jobs:
       jobDisplayName: "Tests: Helix ARM64 matrix"
       agentOs: Linux
       timeoutInMinutes: 480
+      useHostedUbuntu: false
       steps:
       - script: ./eng/build.sh --ci --nobl --pack --arch arm64
                 /p:ASPNETCORE_TEST_LOG_DIR=artifacts/log