formula_installer: add tap_audit_exception stub

`FormulaInstaller` calls `audit_installed` at install time, which
invokes methods in `FormulaCellarChecks`. One of these methods makes a
call to `tap_audit_exception` (cf. #11750), but this method isn't
visible in `FormulaInstaller`.

Instead of trying to replicate the logic of `tap_audit_exception` in
`FormulaAuditor` (or trying to initialise an instance of one to make the
call to `FormulaAuditor`'s implementation of it), let's just implement a
stub that assumes an exception always exists.

I'll need to think a bit about whether this is the right fix for this,
but currently the missing method error is blocking PRs in Homebrew/core,
so let's go with this for now. [1]

[1] e.g. Homebrew/homebrew-core#81388, Homebrew/homebrew-core#81582

Library/Homebrew/formula_installer.rb

@@ -1196,6 +1196,12 @@ class FormulaInstaller
     super
   end
 
+  # This is a stub for calls made to this method at install time.
+  # Exceptions are correctly identified when doing `brew audit`.
+  def tap_audit_exception(*)
+    true
+  end
+
   def self.locked
     @locked ||= []
   end