build: Add missing guix-gc.timer file to binary tarball.

Partially fixes <https://issues.guix.gnu.org/65117>.

* nix/local.mk (nodist_systemdservice_DATA): Add etc/guix-gc.timer.

nix/local.mk

@@ -153,8 +153,9 @@ systemdservicedir = $(libdir)/systemd/system
 nodist_systemdservice_DATA =			\
   etc/gnu-store.mount				\
   etc/guix-daemon.service			\
-  etc/guix-publish.service			\
-  etc/guix-gc.service
+  etc/guix-gc.service				\
+  etc/guix-gc.timer				\
+  etc/guix-publish.service
 
 etc/%.mount: etc/%.mount.in	\
 			 $(top_builddir)/config.status