Make store_artifacts optional

[nanodeath]

Describe Request:

I'm using this orb to build an artifact that's then persisted to a workspace for later processing by itch.io's butler. I have steps that look like this:

    steps:
      - checkout
      - unity/prepare-env
      - unity/build:
          build-name: MyGameWin64
          build-target: StandaloneWindows64
      - persist_to_workspace: 
          root: Builds
          paths:
            - StandaloneWindows64

However, unity/build automatically stores artifacts, using up valuable space in my free CircleCI account. So...it'd be cool if that was optional.

Examples:

Supporting Documentation Links:

[EricRibeiro]

Feature released in v1.1.0.