Ensure qemu-user-static is available

This allows running arm64 binaries in the chroot on the x64 host.

Author: sbomer

eng/pipelines/dotnet-buildtools-prereqs-mariner.yml

@@ -25,3 +25,5 @@ stages:
     internalProjectName: ${{ variables.internalProjectName }}
     publicProjectName: ${{ variables.publicProjectName }}
     linuxAmdBuildJobTimeout: 360
+    customBuildInitSteps:
+    - template: /eng/pipelines/steps/install-cross-build-prereqs.yml