brew audit: fails on Linux if tap has any casks

Running `brew audit --tap=[TAP]` fails on Linux (including GitHub Actions CI) if the tap has any caksks.

Library/Homebrew/dev-cmd/audit.rb

@@ -204,6 +204,7 @@ module Homebrew
     cask_results = if audit_casks.empty?
       {}
     else
+      require "cask/cmd/abstract_command"
       require "cask/cmd/audit"
 
       Cask::Cmd::Audit.audit_casks(