dashboard: run Plan 9 builder on n1-highcpu-4

Updates golang/go#12032.

Change-Id: I7239115b2e0e55f7fc1e5cdf24aa7e314f3bddac
Reviewed-on: https://go-review.googlesource.com/13077
Reviewed-by: Andrew Gerrand <[email protected]>

Author: 0intro

dashboard/builders.go

@@ -526,7 +526,7 @@ func init() {
 		// single-core Plan 9. It will see 2 virtual cores and
 		// only use 1, but we hope that 1 will be more powerful
 		// and we'll stop timing out on tests.
-		machineType: "n1-highcpu-2",
+		machineType: "n1-highcpu-4",
 
 		NumTestHelpers: 5, // slow
 	})