worker/server: support for env_file step parameter

[plyr4]

Description

support for shared env file step source would be cool

steps:
  - name: write-env
    commands:
      - echo "FOO=bar" >> $VELA_BUILD_WORKSPACE/.env-new
  - name: use-env
    env_file: .env-new
    commands:
      - echo $FOO

that would echo "bar"

[plyr4]

maybe this is useless if we end up doing shared volumes with the output driver VELA_ENV