dev/ci: retry candidate image pushes automatically

Created by: bobheadxi

We run into intermittent flakes when pushing images:

eb6b01329ebe: Layer already exists
unknown blob

Poking around the internet this is just a flake that's kind of outside our control. To mitigate, we set a special exit code on push failures that we just retry on

Test plan

main-dry-run: https://buildkite.com/sourcegraph/sourcegraph/builds/144965