Add forward slash to directory prefix

What does this MR do and why?

Did a random check on delete-failed-test-resources job in Staging today and discovered that last week job wasn't able to download resource files from GCS. When I tried the rake task locally I noticed that providing prefix staging alone resulted in the job trying to pull from all directories with name including staging (e.g. staging-ref, staging-canary, staging). Adding the / helps the task to grab files from only staging/ directory. So adding that so I can trigger the job manually and see if it works as expected or anything else needs fixing.

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