Use more machines for windows (#2864)

Allocate more windows machines since we usually see long wait time on
them

buildkite/instances.yml

diff --git a/buildkite/instances.yml b/buildkite/instances.yml
index f80e803..8895fb9 100644
--- a/buildkite/instances.yml
+++ b/buildkite/instances.yml
@@ -26,7 +26,7 @@
   initial_delay: 60
 instance_groups:
   - name: bk-docker
-    count: 180
+    count: 160
     project: bazel-untrusted
     service_account: buildkite@bazel-untrusted.iam.gserviceaccount.com
     image_family: bk-docker
@@ -71,7 +71,7 @@
     zone: us-central1-c
     boot_disk_type: hyperdisk-balanced
   - name: bk-windows
-    count: 80
+    count: 100
     project: bazel-untrusted
     service_account: buildkite@bazel-untrusted.iam.gserviceaccount.com
     image_family: bk-windows