fix(docs-infra): fix preview server periodic clean-up (#25671)

Includes the following fixes:

- Fix cron entry format for clean-up script.
  Crontabs in `/etc` should not have a user field. No idea why it used
  to work before, but it started giving errors recently:
  `/bin/sh: root: not found`.

- Set required env variable in clean-up script. (Broken in cc6f36a9d.)
  This was producing the following error:
  `ERROR: Missing required environment variable 'AIO_CIRCLE_CI_TOKEN'!`

- Use the correct path for downloads to be removed. (Broken in cc6f36a9d.)

PR Close #25671

aio/aio-builds-setup/dockerbuild/scripts-js/lib/clean-up/build-cleaner.ts

@@ -122,6 +122,6 @@ export class BuildCleaner {
     this.logger.log(`Existing downloads: ${existingDownloads.length}`);
     this.logger.log(`Removing ${toRemove.length} download(s): ${toRemove.join(', ')}`);
 
-    toRemove.forEach(filePath => shell.rm(filePath));
+    toRemove.forEach(filePath => shell.rm(path.join(this.downloadsDir, filePath)));
   }
 }