Fix release script version calculation (#1741)

Co-authored-by: David Simansky <[email protected]>

Author: knative-prow-robot

hack/release.sh

@@ -18,10 +18,10 @@
 # at https://github.com/knative/hack
 
 source $(dirname $0)/../vendor/knative.dev/hack/release.sh
-source $(dirname $0)/build-flags.sh
 
 function build_release() {
   # Env var exported by hack/build-flags.sh
+  source $(dirname $0)/build-flags.sh
   local ld_flags="${KN_BUILD_LD_FLAGS:-}"
 
   export CGO_ENABLED=0