Making file writable

[Tomás Ortín Fernández]

I'm trying to pack the updated version of rubocop (1.4.2). The problem is that 
a patch must be applied, and it affects two files that aren't writable, 
.rubocop.yml and .rubocop_todo.yml. I've tried making it writable after 
unpacking:

(arguments ...
...
(add-after 'unpack 'make-yml-files-writable
   (lambda _
      (let ((yml-files (find-files "\\.yml$")))
         (for-each make-file-writable yml-files))
      #t))
...)

I've tried as well doing it with a snippet:

(source ...
...
(modules '((guix build utils)))
   (snippet
     '(begin
         (for-each (make-file-writable (find-files "\\.yml$")))
         #t))
...)
However, I haven't been successful and the files remain non-writable when the 
patch is applied. What am I doing wrong?

Tomás