Propagate step environment to job environment

[lox]

We weren't correctly propagating environment from steps into the job environment.

My test pipeline was:

steps:
  - label: "env test"
    command: "env | grep -e '^TEST_ENV'"
    env: 
      TEST_ENV: "llamas"

Closes #40.

[tduffield]

I can confirm that fixes it on my end as well! 🙇