Set reasonable defaults for Nix

For context: https://jackson.dev/post/nix-reasonable-defaults/

hosts/desktop.nix

@@ -47,6 +47,11 @@ with lib.my;
   ];
 
   nix.settings = {
+    connect-timeout = 5;
+    log-lines = 25;
+    min-free = 128000000;
+    max-free = 1000000000;
+    fallback = true;
     auto-optimise-store = true;
     substituters = [
       "http://odyssey.mesh.vimium.net"

hosts/helios/default.nix

@@ -8,7 +8,6 @@ with lib.my;
   ];
 
   boot.loader.grub.enable = true;
-  boot.loader.grub.version = 2;
   boot.loader.grub.device = "/dev/sda";
   boot.loader.grub.zfsSupport = true;