Fix new notebook CI (#2434)

I missed this while migrating the test to it's own file.

scripts/ci/pr-execute-notebooks.py

@@ -42,7 +42,7 @@
 
 args = ["tox", "--", "--write"]
 if changed_notebooks and not config_changed:
-  args.extend(changed_notebooks.split("\n"))
+  args.extend(changed_notebooks)
 
 is_fork = os.environ["PR_REPOSITORY"] != os.environ["GITHUB_REPOSITORY"]