Rebalance Linux vs Windows VMs (#2830)

The Windows queue is really backed up right now, and the Linux queue is
underutilized, so we should rebalance the VM distribution.

buildkite/instances.yml

diff --git a/buildkite/instances.yml b/buildkite/instances.yml
index 26942ff..bcbcb82 100644
--- a/buildkite/instances.yml
+++ b/buildkite/instances.yml
@@ -26,7 +26,7 @@
   initial_delay: 60
 instance_groups:
   - name: bk-docker
-    count: 140
+    count: 120
     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: 40
+    count: 60
     project: bazel-untrusted
     service_account: buildkite@bazel-untrusted.iam.gserviceaccount.com
     image_family: bk-windows