cmd/coordinator: disable netbsd builds for now

They're not working yet.

Updates golang/go#8643

Change-Id: I3b9b91bcd2071a59a232ea936d7e0495adecba9a
Reviewed-on: https://go-review.googlesource.com/21499
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/coordinator/coordinator.go

@@ -374,6 +374,9 @@ func mayBuildRev(rev builderRev) bool {
 	if inStaging && numCurrentBuilds() != 0 {
 		return false
 	}
+	if strings.Contains(rev.name, "netbsd") {
+		return false
+	}
 	if dashboard.Builders[rev.name].IsReverse {
 		return reversePool.CanBuild(rev.name)
 	}