gnu: wla-dx: Use G-expressions.

* gnu/packages/assembly.scm (wla-dx)[arguments]:
Rewrite as G-expressions.

gnu/packages/assembly.scm

@@ -359,8 +359,9 @@ package for the Game Boy and Game Boy Color.  It consists of:
     (build-system cmake-build-system)
     (native-inputs (list python-sphinx)) ; to generate man pages
     (arguments
-     `(#:phases
-       (modify-phases %standard-phases
+     (list
+      #:phases
+      #~(modify-phases %standard-phases
           (add-before 'check 'copy-tests-to-build-directory
             (lambda _
               (copy-recursively "../source/tests" "tests")))