Move Nix files back to the root of the project

hosts/t490/services/cron.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+
+{
+  services.cron = {
+    enable = true;
+
+    systemCronJobs = [
+      "*/15 * * * * opdavies ${pkgs.imapfilter}/bin/imapfilter -c ~/Code/email-filters/config.lua"
+      "0 8,20 * * * opdavies ${pkgs.isync}/bin/mbsync -a"
+    ];
+  };
+}