fix mz dependencies

[mironal]

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• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

I got an error Cannot find module 'mz/child_process' with electron-packager v10.0.0.

I found mz/child_process to be used in following code.

https://github.com/electron-userland/electron-packager/blob/2a9374e027494d97a984b469c7540cf501be2a08/prune.js#L3

And I moved mz from devDependencies to dependencies in package.json.

[welcome]

Thanks for opening a pull request!

Here are some highlighted action items that will help get it across the finish line, from the
pull request guidelines:

• Follow the JavaScript coding style.
• Run npm run lint locally to catch formatting errors earlier.
• Document any user-facing changes in NEWS.md and other docs.
• Include tests when adding/changing behavior.

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[malept]

Thanks for catching that. In the future, it would be nice if the summary was filled in so it's more obvious why the changes are warranted.

[welcome]

Thanks for your contribution! 🎉