In general would say make sitemap.xml a dynamic handler.
It could just consult your database to find a list of URLs and just output the XML on the fly. Potentially caching it to save doing it every time.
... or it could download the actual XML from Cloud Storage, or even the BlobStore. Whatever offline process creates the sitemap, just uploads it there when done.
You could have your script recreate the file and then 'deploy' your app, to upload the to the static file storage area, but regular deploys are not ideal, it could cause technically unnesseraly overhead. Might need to carefully manage versions to avoid leaving lots of versions lying around.