services: virtual-build-machine: Add periodic GC job.

* gnu/services/virtualization.scm (%virtual-build-machine-operating-system):
Add ‘periodic-gc’ service.

Change-Id: Id5bde1ee3f8e1e549e231375e776d3201ec9e0b5

gnu/services/virtualization.scm

@@ -36,6 +36,7 @@
   #:use-module (gnu services base)
   #:use-module (gnu services configuration)
   #:use-module (gnu services dbus)
+  #:use-module (gnu services mcron)
   #:use-module (gnu services shepherd)
   #:use-module (gnu services ssh)
   #:use-module (gnu services)
@@ -1209,6 +1210,11 @@ authpriv.*;auth.info                    /var/log/secure
                               (openssh-configuration
                                (openssh openssh-sans-x)))
 
+                     ;; Run GC once per hour.
+                     (simple-service 'perdiodic-gc mcron-service-type
+                                     (list #~(job "12 * * * *"
+                                                  "guix gc -F 2G")))
+
                      (modify-services %base-services
                        ;; By default, the secret service introduces a
                        ;; pre-initialized /etc/guix/acl file in the VM.  Thus,