Enable unsigned installers

[johnwparent]

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Assigning a negative value to the GPT4ALL_SIGN_INSTALL CMake option will now allow for the production of unsigned installers.

This is also the default behavior as singing is not enabled unless explicitly requested.

Describe your changes

Issue ticket number and link

Checklist before requesting a review

• [x] I have performed a self-review of my code.
• [ ] If it is a core feature, I have added thorough tests.
• [ ] I have added thorough documentation for my code.
• [ ] I have tagged PR with relevant project labels. I acknowledge that a PR without labels may be dismissed.
• [ ] If this PR addresses a bug, I have provided both a screenshot/video of the original bug and the working solution.

Demo

Steps to Reproduce

Notes

cc @cebtenzzre