sorbet: Start typechecking install.rb and reinstall.rb

[issyl0]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
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• You can probably tell from the branch name that I had greater ambitions for tonight's Sorbet session than just two files. But I'm tired and it's still two files done!
• This moves install.rb and reinstall.rb to true.