Added CI step to trigger canary build
Added CI step to trigger canary build

refs https://github.com/TryGhost/Ghost/commit/84bfbae282f887a4b28fde206eb1d9b3390df0c8

- if Admin is pushed to after Ghost, the canary build won't contain
  Admin changes, so we need to trigger another canary build
- this commit adds a final step to the CI flow that will trigger the
  Ghost canary build if all steps were successful

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost