specify the local and remote refs for the push

ci-tools/update-nixpkgs

@@ -18,7 +18,7 @@ nix-build -A morph -o result-after
 diff=$(nix --extra-experimental-features nix-command store diff-closures ./result-before/ ./result-after/)
 
 git commit -am "bump nixpkgs version"
-git push origin
+git push origin "${TARGET_BRANCH}:${TARGET_BRANCH}"
 
 BODY="{
     \"id\": ${CI_PROJECT_ID},