formula_auditor: Having HOMEBREW_PREFIX in keg_only reasons is bad

- Fixes issue 14996.
- The API JSON is generated with a `/usr/local` Homebrew prefix, but
  frequently now users have `/opt/homebrew` as their prefix. Since
  formulae `keg_only` reasons are generated by the API, this can lead
  to the confusing messaging that follows:

```
socket_vmnet is keg-only, which means it was not symlinked into /opt/homebrew,
because /usr/local/bin is often writable by a non-admin user.
```

- With this change, that formula as it is now will fail `brew audit`,
  prompting to remove the prefix reference.

```
❯ HOMEBREW_NO_INSTALL_FROM_API=1 brew audit --strict socket_vmnet
socket_vmnet:
  * `keg_only` reason should not include `HOMEBREW_PREFIX` as it provides confusing output.
Error: 1 problem in 1 formula detected
```

Library/Homebrew/formula_auditor.rb

@@ -466,6 +466,16 @@ module Homebrew
               "They must not be upgraded to version 7.11 or newer."
     end
 
+    def audit_keg_only_reason
+      return unless @core_tap
+      return unless formula.keg_only?
+
+      keg_only_message = text.to_s.match(/keg_only\s+["'](.*)["']/).captures&.first
+      return unless keg_only_message&.include?("HOMEBREW_PREFIX")
+
+      problem "`keg_only` reason should not include `HOMEBREW_PREFIX` as it provides confusing output."
+    end
+
     def audit_versioned_keg_only
       return unless @versioned_formula
       return unless @core_tap