Extracted Canary workflow to template
Extracted Canary workflow to template

refs https://github.com/TryGhost/actions/commit/4aee97472e172defe59258212ddb6e40e45591ca

- the referenced commit copied the workflow file to our shared templates
  repository
- this commit switches to using that template
- this should help reduce duplication when we're doing v5

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost