dev/ci: backend integration tests should use candidate image

Created by: bobheadxi

We spend ages on sourcegraph/server builds - https://buildkite.com/sourcegraph/sourcegraph/builds/109471#f0a44a31-0ff8-450f-8065-1147fd361582/1810-1811 + the previous commands running binary builds and downloads for various dependencies. The candidate image is used in the various QA tests, we should use it here too.