services: greetd: Use mkdir-p in activation script.

Fixes a bug introduced in ee0d1b144c where
reconfiguring a system with ‘greetd-service-type’ would fail with:

  guix system: error: mkdir: File exists "/run/user".

* gnu/services/base.scm (greetd-run-user-activation): Replace mkdir with mkdir-p.

Change-Id: I030d1f57c3292c518b3f17061f75258e8a72141c

gnu/services/base.scm

@@ -3875,9 +3875,11 @@ to handle."
            pam))))))
 
 (define (greetd-run-user-activation config)
-  #~(let ((d "/run/user"))
-      (mkdir d #o755)
-      (chmod d #o755)))
+  #~(begin
+      (use-modules (guix build utils))
+      (let ((d "/run/user"))
+        (mkdir-p d)
+        (chmod d #o755))))
 
 (define (greetd-shepherd-services config)
   (map