[artifacts] Switch to OAUTH GCS FullControl scope.

This is the scope the luci-token-server has whitelisted for this role.

Change-Id: I964dc3e5876152e209eb15cf7891198931297f0d
diff --git a/artifacts/artifacts.go b/artifacts/artifacts.go
index f77dd02..1499d0a 100644
--- a/artifacts/artifacts.go
+++ b/artifacts/artifacts.go
@@ -20,7 +20,7 @@
 
 // NewClient creates a new Client.
 func NewClient(ctx context.Context, opts auth.Options) (*Client, error) {
-	opts.Scopes = append(opts.Scopes, storage.ScopeReadWrite)
+	opts.Scopes = append(opts.Scopes, storage.ScopeFullControl)
 	client, err := gcs.NewClient(ctx, opts)
 	if err != nil {
 		return nil, err