Shrink untrusted ARM64 Docker pools (#2285)

Our current quota only allows 80 cores.

Progress towards
#2272

Author: fweikert

buildkite/instances.yml

@@ -44,7 +44,7 @@ instance_groups:
     image_family: bk-docker
     metadata_from_file: startup-script=startup-docker-pdssd.sh
   - name: bk-docker-arm64
-    count: 20
+    count: 8
     project: bazel-untrusted
     service_account: [email protected]
     image_family: bk-docker-arm64
@@ -53,7 +53,7 @@ instance_groups:
     zone: us-central1-c
     boot_disk_type: hyperdisk-balanced
   - name: bk-testing-docker-arm64
-    count: 10
+    count: 2
     project: bazel-untrusted
     service_account: [email protected]
     image_family: bk-testing-docker-arm64