CLI/Storyshots: Specify custom sb extract Chromium exe

[brianblakely]

Issue: #13724

What I did

Added environment variable SB_CHROMIUM_PATH, which enables a way to customize the Chromium executable used by sb extract and StoryShots, without an automatic download. StoryShots' existing config-based way of doing this will still work and will take precedence when both are used.

How to test

1. Download Chromium (you can use Puppeteer's copy in node_modules).
2. Set the var. It can be an absolute or relative path to the executable.
3. Run sb extract / a StoryShots + Puppeteer test.

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

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[shilman]

Looking great @brianblakely -- nice fix 👏 👏 👏