cmd/buildstats: fix breaking API change in dependency

CL https://code-review.googlesource.com/#/c/9567/ unexported
bigquery.ValueList, which broke the code in this project. Switch the
buildstats code per the suggestion in the linked commit.

Fixes golang/go#18764.

Change-Id: I55b748f65732d5627d966623cd3b0e73223b16be
Reviewed-on: https://go-review.googlesource.com/35572
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/buildstats/buildstats.go

@@ -15,10 +15,9 @@ import (
 	"reflect"
 	"time"
 
-	"golang.org/x/build/types"
-
 	"cloud.google.com/go/bigquery"
 	"cloud.google.com/go/datastore"
+	"golang.org/x/build/types"
 	"google.golang.org/api/iterator"
 )
 
@@ -60,7 +59,7 @@ func sync(ctx context.Context) {
 	if err != nil {
 		log.Fatalf("Read: %v", err)
 	}
-	var values bigquery.ValueList
+	var values *[]bigquery.Value
 	err = it.Next(&values)
 	if err == iterator.Done {
 		log.Fatalf("No result.")
@@ -68,7 +67,7 @@ func sync(ctx context.Context) {
 	if err != nil {
 		log.Fatalf("Next: %v", err)
 	}
-	t, ok := values[0].(time.Time)
+	t, ok := (*values)[0].(time.Time)
 	if !ok {
 		log.Fatalf("not a time")
 	}