Forget about assignee for a minute and try to generate the body

ci-tools/update-nixpkgs

 #!/usr/bin/env nix-shell
-#!nix-shell -i bash -p nixUnstable git openssh curl
+#!nix-shell -i bash -p nixUnstable git openssh curl python3
 
 # ^^
 # we get nixUnstable for the diff-closures command, mostly.
@@ -58,21 +58,22 @@ if git diff --exit-code origin/develop...; then
 fi
 
 # nix-build -A morph -o result-after
-# diff=$(nix --extra-experimental-features nix-command store diff-closures ./result-before/ ./result-after/)
+# DIFF=$(nix --extra-experimental-features nix-command store diff-closures ./result-before/ ./result-after/)
+DIFF="Hello world"
 
 git commit -am "bump nixpkgs version"
 git push --force origin "${TARGET_BRANCH}:${TARGET_BRANCH}"
 
-BODY=$(cat <<EOF
-{
-    "id": ${CI_PROJECT_ID},
-    "source_branch": "${CI_COMMIT_REF_NAME}",
-    "target_branch": "${TARGET_BRANCH}",
-    "remove_source_branch": true,
-    "title": "WIP: ${CI_COMMIT_REF_NAME}",
-    "assignee_id":"jcalderone"
-}
-EOF
-    )
-
-curl -X POST --data "${BODY}" --header "Content-Type: application/json" --header "PRIVATE-TOKEN: ${UPDATE_NIXPKGS_PRIVATE_TOKEN}" "https://${HOST}/api/v4/projects/${CI_PROJECT_ID}/merge_requests"
+BODY=$(python3 -c '
+import os, sys, json
+print(json.dumps({
+    "id": os.environ["CI_PROJECT_ID"],
+    "source_branch": os.environ["CI_COMMIT_REF_NAME"],
+    "target_branch": os.environ["TARGET_BRANCH"],
+    "remove_source_branch": True,
+    "title": f"WIP: {os.environ["CI_COMMIT_REF_NAME"]}",
+    "description": sys.argv[1],
+}))
+' "${DIFF}")
+
+echo curl -X POST --data "${BODY}" --header "Content-Type: application/json" --header "PRIVATE-TOKEN: ${UPDATE_NIXPKGS_PRIVATE_TOKEN}" "https://${HOST}/api/v4/projects/${CI_PROJECT_ID}/merge_requests"