Fix building many things in nix.

Nix 2.30.0 (2025-07-07) changed the build directory from /tmp to /nix/var/nix/builds which broke a number of builds because my ZFS datasets were utf8only. This adds a role to mount tmpfs to /nix/var/nix/builds to restore the old behavior.

nix/configuration/roles/build_in_ram/default.nix

@@ -0,0 +1,35 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  imports = [ ];
+
+  options.me = {
+    build_in_ram.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install build_in_ram.";
+    };
+  };
+
+  config = lib.mkIf config.me.build_in_ram.enable (
+    lib.mkMerge [
+      {
+        # Nix 2.30.0 (2025-07-07) changed the build directory from /tmp to /nix/var/nix/builds which broke a number of builds because my ZFS datasets were utf8only.
+        fileSystems."/nix/var/nix/builds" = {
+          device = "tmpfs";
+          fsType = "tmpfs";
+          options = [
+            "size=40G" # adjust for your situation and needs
+            "mode=700"
+          ];
+        };
+      }
+    ]
+  );
+}