Fix post_install: use system cp -f to handle existing fonts

Ruby's cp raises EPERM if the target file already exists.
Using system "cp", "-f" overwrites gracefully.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.fontfoldercleanup/create_homebrew_formula.py

@@ -121,10 +121,10 @@ class Font{class_name} < Formula
     user_fonts.mkpath
 
     Dir.glob(share/"fonts/truetype/*.ttf").each do |f|
-      cp f, user_fonts
+      system "cp", "-f", f, user_fonts.to_s
     end
     Dir.glob(share/"fonts/opentype/*.otf").each do |f|
-      cp f, user_fonts
+      system "cp", "-f", f, user_fonts.to_s
     end
   end