Support overriding and adding environment variables

[Hobart2967]

Acceptance Criteria

• It is possible to selectively set environment variables when running job tasks.
• It is possible to generally set environment variables when skwid.

Example

env:
  MY_VAR: "A_VALUE"

tasks:
  init:
    description: Initializes the repository
    jobs:
      - name: Install packages
        type: command
        command: yarn
        env: 
          FORCE_LOCAL: 1