Use `is_read_only` to allow/forbid to update Worker Activity

Instead of checking for self.args.dev, we should check for is_read_only because being in read_only mode forbids us from updating worker activities. https://gitlab.teklia.com/workers/base-worker/-/blob/5781d8859aa8ddef8da32eb0c995c2de27b626fe/arkindex_worker/worker/init.py#L139