running build-flags.sh as part of release (#66)

Co-authored-by: Paul S. Schweigert <paulschw@us.ibm.com>

Author: knative-prow-robot

hack/build-flags.sh

@@ -20,6 +20,7 @@
 
 source $(dirname "$0")/global_vars.sh
 source $(dirname "$0")/../vendor/knative.dev/hack/release.sh
+source $(dirname $0)/build-flags.sh
 
 function build_release() {
   export GO111MODULE=on