formula: fix latest_version_installed? for HOMEBREW_JSON_CORE

[Rylan12]

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Follow-up to #11715

If HOMEBREW_JSON_CORE is set, brew cleanup fails:

$ brew cleanup
Error: no implicit conversion of nil into String
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/pathname.rb:350:in `initialize'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/pathname.rb:350:in `new'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/pathname.rb:350:in `+'
/usr/local/Homebrew/Library/Homebrew/formula.rb:2351:in `versioned_prefix'
/usr/local/Homebrew/Library/Homebrew/formula.rb:597:in `prefix'
/usr/local/Homebrew/Library/Homebrew/formula.rb:524:in `latest_version_installed?'
/usr/local/Homebrew/Library/Homebrew/formula.rb:2257:in `eligible_kegs_for_cleanup'
/usr/local/Homebrew/Library/Homebrew/cleanup.rb:237:in `cleanup_formula'
/usr/local/Homebrew/Library/Homebrew/cleanup.rb:190:in `block in clean!'
/usr/local/Homebrew/Library/Homebrew/cleanup.rb:189:in `each'
/usr/local/Homebrew/Library/Homebrew/cleanup.rb:189:in `clean!'
/usr/local/Homebrew/Library/Homebrew/cmd/cleanup.rb:58:in `cleanup'
/usr/local/Homebrew/Library/Homebrew/brew.rb:122:in `<main>'

This is because the updates to Formula#latest_version_installed? from #11715 did not handle the case where a formula wasn't found in the /api/versions.json file (which would happen for any non-core formula.

Now, if the formula isn't listed, default back to the old way of using latest_installed_prefix (which is still done if HOMEBREW_JSON_CORE is unset.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense!