fix release script

The release script had a typo

Author: manuelnaranjo

.github/workflows/release_prep.sh

@@ -28,7 +28,7 @@ mkdir -p ${PREFIX}/tools
 
 PREFIX=$PREFIX ${SCRIPT_DIR}/generate_tools_versions.sh
 
-PREBUILTS=$(jq \
+INTEGRITY=$(jq \
   --from-file .github/workflows/integrity.jq \
   --arg PREFIX "bazeldnf-${GITHUB_REF_NAME}-" \
   --slurp \
@@ -38,7 +38,7 @@ PREBUILTS=$(jq \
 cat >${PREFIX}/tools/integrity.bzl <<EOF
 "Generated during release by release_prep.sh, using integrity.jq"
 
-PREBUILTS = ${PREBUILTS}
+INTEGRITY = ${INTEGRITY}
 
 EOF