ox-odt: Get write permissions on styles.xml

* lisp/ox-odt.el (org-odt-template): The file `styles.xml' might be
copied from a location where it is read-only.  Since we need to modify
it (and its copied under /tmp, so we can modify it), we need to ensure
we have write persmissions on it.  Set mode to 600.

This is especially needed for projects like Guix, where all system
files are stored in a read-only location.

TINYCHANGE

lisp/ox-odt.el

@@ -1383,6 +1383,8 @@ original parsed data.  INFO is a plist holding export options."
 
     ;; create a manifest entry for styles.xml
     (org-odt-create-manifest-file-entry "text/xml" "styles.xml")
+    ;; Ensure we have write permissions to this file.
+    (set-file-modes (concat org-odt-zip-dir "styles.xml") #o600)
 
     ;; FIXME: Who is opening an empty styles.xml before this point?
     (with-current-buffer