During CI, use just 1 cpu for QEMU builds

cirruslabs/cirrus-ci-docs#741

Author: asomers

.cirrus.yml

@@ -52,12 +52,16 @@ task:
 
 # Use cross for QEMU-based testing
 # cross needs to execute Docker, so we must use Cirrus's Docker Builder task.
-docker_builder:
+compute_engine_instance:
+  image_project: cirrus-images
+  image: family/docker_builder
+  platform: linux
+  cpu: 1                            # Since QEMU will only use 1 thread
+  memory: 4G
   env:
     RUST_TEST_THREADS: 1            # QEMU works best with 1 thread
     HOME: /tmp/home
     PATH: $HOME/.cargo/bin:$PATH
-      #cpu: 1 docker_builder doesn't support "cpu"?
   matrix:
     - name: Linux aarch64
       env: