Fix brew untap with HOMEBREW_INSTALL_FROM_API

Library/Homebrew/cmd/untap.rb

@@ -27,6 +27,7 @@ module Homebrew
     args.named.to_installed_taps.each do |tap|
       odie "Untapping #{tap} is not allowed" if tap.core_tap? && !Homebrew::EnvConfig.install_from_api?
 
+      if !Homebrew::EnvConfig.install_from_api? || (!tap.core_tap? && tap != "homebrew/cask")
         installed_tap_formulae = Formula.installed.select { |formula| formula.tap == tap }
         installed_tap_casks = Cask::Caskroom.casks.select { |cask| cask.tap == tap }
 
@@ -44,6 +45,7 @@ module Homebrew
             EOS
           end
         end
+      end
 
       tap.uninstall manual: true
     end