Install cross build prereqs for Ubuntu pipeline (#1106)

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

@@ -28,3 +28,5 @@ extends:
         internalProjectName: ${{ variables.internalProjectName }}
         publicProjectName: ${{ variables.publicProjectName }}
         linuxAmdBuildJobTimeout: 360
+        customBuildInitSteps:
+        - template: /eng/pipelines/steps/install-cross-build-prereqs.yml@self

src/ubuntu/22.04/cross/arm/Dockerfile

@@ -5,12 +5,6 @@ ARG ROOTFS_DIR=/crossrootfs/arm
 RUN git config --global user.email builder@dotnet-buildtools-prereqs-docker && \
     git clone --depth 1 --single-branch https://github.com/dotnet/arcade /scripts
 
-RUN apt-get update \
-    && apt-get install -y \
-        qemu \
-        qemu-user-static \
-    && rm -rf /var/lib/apt/lists/*
-
 # Build the rootfs
 RUN /scripts/eng/common/cross/build-rootfs.sh arm jammy lldb14 --skipunmount