self: Mark trivial "-modules" derivations as non-substitutable.

The resulting nar takes ~500KiB and it's quicker to build it locally
than to download it.

* guix/self.scm (node-source+compiled): Pass #:options to
'computed-file'.

guix/self.scm

@@ -124,7 +124,11 @@ NODE's modules, under their FHS directories: share/guile/site and lib/guile."
           (symlink #$(node-compiled node) object))))
 
   (computed-file (string-append (node-name node) "-modules")
-                 build))
+                 build
+                 #:options '(#:local-build? #t
+
+                             ;; "Building" it locally is faster.
+                             #:substitutable? #f)))
 
 (define (node-fold proc init nodes)
   (let loop ((nodes nodes)