add(tmp): make /tmp an actual tmpfs

modules/nixos/system/tmp/default.nix

@@ -0,0 +1,25 @@
+{
+  config,
+  lib,
+  namespace,
+  ...
+}:
+with lib;
+with lib.${namespace};
+let
+  cfg = config.${namespace}.system.tmpfs;
+in
+{
+  options.${namespace}.system.tmpfs = with types; {
+    enable = mkBoolOpt false "Whether or not to use tmpfs for /tmp.";
+  };
+
+  config = mkIf cfg.enable {
+    boot.tmp = {
+      cleanOnBoot = true;
+      tmpfsSize = "50%";
+      useTmpfs = true;
+    };
+  };
+
+}