engine: can't render step outputs with a `-` in them

[abelanger5]

For example, this will fail because the - is not permitted as a field reference:

name: example-workflow
version: 0.1.0
triggers:
  events:
    - user:create
jobs:
  post-user-create:
    steps:
      - id: example-step
        action: default:action1
        with:
          object: '{{ .input.json }}'
      - id: example-step-2
        action: default:action2
        with:
          object: '{{ .steps.example-step.json }}'