head-node: Update WP3 jobset.

* head-node.scm: Modify cuirass specs.

head-node.scm

@@ -257,7 +257,7 @@
                    (name 'guix-hpc)
                    (url
                     "https://gitlab.inria.fr/numpex-pc5-wp3/guix-hpc.git")
-                   (branch "master"))
+                   (branch "testing"))
                   #$(guix-channel "master"))))))
 
 (define this-file