bump sha256 for latest upstream image
Created by: davejrt
Fixes manifest for quay.io/prometheus/busybox-linux-amd64:latest@sha256:de4af55df1f648a334e16437c550a2907e0aed4f0b0edf454b0b215a9349bdbb not found: manifest unknown: manifest unknown
https://buildkite.com/sourcegraph/sourcegraph/builds/119871#9a9763bf-2253-4a39-9715-bff2843e5555/71-144