dev/ci: use candidate image in backend int. tests
Created by: bobheadxi
Currently, the backend integration tests builds a fresh server image to run tests against. There is another job that does this, the candidate image build - this change adds a dependency between the two steps and changes the integration tests to use the candidate image instead.
- Before: ~15mins
- After: ~3mins (after server builds, which is still ~11mins sadly, but this is a healthy amount of compute time saved IMO and makes it easier to replicate tests locally e.g. #25409 )
Closes #25344 (closed)