nix-gc: add randomizedDelaySec option

There's no launchd equivalent to this option, so this is a no-op on
Darwin.

modules/services/nix-gc.nix

@@ -81,6 +81,18 @@ in {
         '';
       };
 
+      randomizedDelaySec = lib.mkOption {
+        default = "0";
+        type = lib.types.singleLineStr;
+        example = "45min";
+        description = ''
+          Add a randomized delay before each garbage collection.
+          The delay will be chosen between zero and this value.
+          This value must be a time span in the format specified by
+          {manpage}`systemd.time(7)`
+        '';
+      };
+
       options = mkOption {
         type = types.nullOr types.str;
         default = null;
@@ -121,6 +133,7 @@ in {
         Unit = { Description = "Nix Garbage Collector"; };
         Timer = {
           OnCalendar = "${cfg.frequency}";
+          RandomizedDelaySec = cfg.randomizedDelaySec;
           Persistent = cfg.persistent;
           Unit = "nix-gc.service";
         };

tests/modules/services/nix-gc/basic.nix

@@ -4,6 +4,7 @@
   nix.gc = {
     automatic = true;
     frequency = "monthly";
+    randomizedDelaySec = "42min";
     options = "--delete-older-than 30d --max-freed $((64 * 1024**3))";
   };
 

tests/modules/services/nix-gc/expected.timer

@@ -4,6 +4,7 @@ WantedBy=timers.target
 [Timer]
 OnCalendar=monthly
 Persistent=true
+RandomizedDelaySec=42min
 Unit=nix-gc.service
 
 [Unit]