Blocks: use placeholder instead of random version in CLI template

[jeherve]

Proposed changes:

Since we have the $$next-version$$ that can be used whenever we need to provide a version number, let's use that instead of a hardcoded version that will never be up-to-date for too long.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

• N/A

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

Not much to test here, but you could try to run jetpack docker wp jetpack scaffold block "thing".

[github-actions]

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  bin/jetpack-downloader test jetpack fix/jetpack-block-cli-reference
  

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[github-actions]

Thank you for your PR!

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Jetpack plugin:

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