home-manager: Add nix.conf

home-manager/configs/castle.nix

@@ -15,6 +15,7 @@
             hostname = "h.nul.ie";
             sshOpts = [ "-4" "-p" "8022" ];
           };
+          nix.config.cores = "16";
         };
 
         programs = {

home-manager/configs/macsimum.nix

@@ -9,6 +9,7 @@
       {
         my = {
           deploy.enable = false;
+          nix.config.cores = "6";
         };
 
         programs = {

home-manager/modules/common.nix

@@ -3,7 +3,7 @@ let
   inherit (builtins) listToAttrs mapAttrs readFile;
   inherit (lib)
     optionalString nameValuePair concatMapStrings concatStringsSep optionalAttrs versionAtLeast
-    mkMerge mkIf mkDefault mkOption;
+    mapAttrsToList mkMerge mkIf mkDefault mkOption;
   inherit (lib.hm) dag;
   inherit (lib.my) mkOpt' dummyOption;
 in
@@ -19,6 +19,11 @@ in
       shell = mkOpt' str null "User's shell (so NixOS or others can set it externally).";
       fishCompletionsFrequency = mkOpt' (nullOr str) "daily" "How often to generate fish completions from manpages.";
 
+      nix = {
+        # TODO: Until https://github.com/nix-community/home-manager/issues/2324
+        config = mkOpt' (attrsOf str) { } "nix.conf options";
+      };
+
       ssh = {
         authKeys = {
           literal = mkOpt' (listOf singleLineStr) [ ] "List of OpenSSH keys to allow";
@@ -47,6 +52,11 @@ in
         isStandalone = !(args ? osConfig);
 
         shell = mkDefault "${config.programs.fish.package}/bin/fish";
+
+        nix.config = {
+          experimental-features = mkDefault "nix-command flakes ca-derivations";
+          max-jobs = mkDefault "auto";
+        };
       };
 
       home.file.".ssh/authorized_keys" = with config.my.ssh.authKeys;
@@ -57,6 +67,10 @@ in
           '';
         };
 
+      xdg.configFile."nix/nix.conf" = mkIf (config.my.nix.config != { }) {
+        text = (concatStringsSep "\n" (mapAttrsToList (k: v: "${k} = ${v}") config.my.nix.config)) + "\n";
+      };
+
       programs = {
         # Even when enabled this will only be actually installed in standalone mode
         # Note: `home-manager.path` is for telling home-manager is installed and setting it in NIX_PATH, which we should