Configure nix inside the machines

Add sane nix defaults for debugging and usability reasons

nixosModules/clanCore/flake-module.nix

@@ -7,6 +7,7 @@
       ./meshnamed
       ./networking.nix
       ./packages.nix
+      ./nix-settings.nix
       inputs.sops-nix.nixosModules.sops
       # just some example options. Can be removed later
       ./vm.nix

nixosModules/clanCore/nix-settings.nix

@@ -0,0 +1,28 @@
+{ lib, ... }:
+# Taken from:
+# https://github.com/nix-community/srvos/blob/main/nixos/common/nix.nix
+{
+  # Fallback quickly if substituters are not available.
+  nix.settings.connect-timeout = 5;
+
+  # Enable flakes
+  nix.settings.experimental-features = [
+    "nix-command"
+    "flakes"
+    "repl-flake"
+  ];
+
+  # The default at 10 is rarely enough.
+  nix.settings.log-lines = lib.mkDefault 25;
+
+  # Avoid disk full issues
+  nix.settings.max-free = lib.mkDefault (3000 * 1024 * 1024);
+  nix.settings.min-free = lib.mkDefault (512 * 1024 * 1024);
+
+  nix.daemonCPUSchedPolicy = lib.mkDefault "batch";
+  nix.daemonIOSchedClass = lib.mkDefault "idle";
+  nix.daemonIOSchedPriority = lib.mkDefault 7;
+
+  # Avoid copying unnecessary stuff over SSH
+  nix.settings.builders-use-substitutes = true;
+}