nix-gc: add persistent option (#5490)

Mirrors the `nix.gc.persistent` option in nixpkgs:
https://search.nixos.org/options?channel=unstable&show=nix.gc.persistent&from=0&size=50&sort=relevance&type=packages&query=nix.gc.persistent

modules/services/nix-gc.nix

@@ -90,6 +90,18 @@ in {
           garbage collector is run automatically.
         '';
       };
+
+      persistent = mkOption {
+        type = types.bool;
+        default = true;
+        example = false;
+        description = ''
+          If true, the time when the service unit was last triggered is
+          stored on disk. When the timer is activated, the service unit is
+          triggered immediately if it would have been triggered at least once
+          during the time when the timer was inactive.
+        '';
+      };
     };
   };
 
@@ -107,6 +119,7 @@ in {
         Unit = { Description = "Nix Garbage Collector"; };
         Timer = {
           OnCalendar = "${cfg.frequency}";
+          Persistent = cfg.persistent;
           Unit = "nix-gc.service";
         };
         Install = { WantedBy = [ "timers.target" ]; };

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

@@ -3,6 +3,7 @@ WantedBy=timers.target
 
 [Timer]
 OnCalendar=monthly
+Persistent=true
 Unit=nix-gc.service
 
 [Unit]