dashboard: expect 1 windows-arm64-mini host.

This will make the dashboard view more consistent.

For golang/go#42604

Change-Id: I90991c9e6c7ba5fc781c6ea1259593048156ca79
Reviewed-on: https://go-review.googlesource.com/c/build/+/333076
Trust: Alexander Rakoczy <[email protected]>
Run-TryBot: Alexander Rakoczy <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: toothrot

dashboard/builders.go

@@ -472,6 +472,7 @@ var Hosts = map[string]*HostConfig{
 		env:                []string{"GOARCH=arm64"},
 		goBootstrapURLTmpl: "https://storage.googleapis.com/$BUCKET/gobootstrap-windows-arm64-f22ec5.tar.gz",
 		IsReverse:          true,
+		ExpectNum:          1,
 	},
 	"host-darwin-10_12": &HostConfig{
 		IsReverse: true,