handle undefined variable in release scripts (#708)

Author: cjerad

scripts/build-and-push-images.sh

@@ -12,7 +12,7 @@ fi
 
 version="$(make -s -f "${makefile_path}" version)"
 platforms="linux/amd64"
-image_repository="${KO_DOCKER_REPO}"
+image_repository="${KO_DOCKER_REPO:-}"
 goproxy="direct|https://proxy.golang.org"
 
 usage=$(cat << EOM

scripts/sync-readme-to-ecr-public.sh

@@ -11,7 +11,7 @@ if ! command -v jq; then
 fi
 
 region="${AWS_REGION:-us-east-1}"
-repo_root="${KO_DOCKER_REPO}"
+repo_root="${KO_DOCKER_REPO:-}"
 version="$(make -s -f "${makefile_path}" version)"
 
 usage=$(cat << EOM