upgrade: fix error checking dependents

[Bo98]

• 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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• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Error: undefined local variable or method `formulae_to_install' for Homebrew:Module
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/upgrade.rb:164:in `block in check_installed_dependents'
/usr/local/Homebrew/Library/Homebrew/cache_store.rb:17:in `use'
/usr/local/Homebrew/Library/Homebrew/upgrade.rb:163:in `check_installed_dependents'
/usr/local/Homebrew/Library/Homebrew/cmd/upgrade.rb:114:in `upgrade'
/usr/local/Homebrew/Library/Homebrew/brew.rb:111:in `<main>'

[MikeMcQuaid]

Thanks for the fix, sorry for the bug @Bo98!