cmd/buildlet, cmd/release: disable pargzip for release builds

Fixes golang/go#19052

Change-Id: Icf2923ea72290bd99b78e1c2fd9675668157b3ac
Reviewed-on: https://go-review.googlesource.com/36910
Reviewed-by: Quentin Smith <quentin@golang.org>
diff --git a/cmd/release/release.go b/cmd/release/release.go
index cf64641..db34d8c 100644
--- a/cmd/release/release.go
+++ b/cmd/release/release.go
@@ -257,6 +257,7 @@
 	if err != nil {
 		return nil, err
 	}
+	bc.SetReleaseMode(true) // disable pargzip; golang.org/issue/19052
 	return bc, nil
 }