let zeitwerk:check report files outside the root directory with absolute paths

This may happen in engines loaded as gems, for example.

Closes #37285.

railties/lib/rails/tasks/zeitwerk.rake

@@ -47,7 +47,7 @@ namespace :zeitwerk do
       eager_load[]
     rescue NameError => e
       if e.message =~ /expected file .*? to define constant \S+/
-        abort $&.sub(/#{Regexp.escape(Rails.root.to_s)}./, "")
+        abort $&.sub(/\A#{Regexp.escape(Rails.root.to_s)}./, "")
       else
         raise
       end