nixos/fstrim: fix overriding the timer interval

The OnCalendar systemd setting is additive. To clear the setting defined
by the pre-existing unit file, it has to be set first to an empty
string, then to the desired value.

(cherry picked from commit 44438e0dcd)

nixos/modules/services/misc/fstrim.nix

@@ -34,7 +34,7 @@ in {
 
     systemd.timers.fstrim = {
       timerConfig = {
-        OnCalendar = cfg.interval;
+        OnCalendar = [ "" cfg.interval ];
       };
       wantedBy = [ "timers.target" ];
     };