add possibility for changing TMPDIR of nix daemon

commented out by default

nixos/configuration.nix

@@ -249,6 +249,8 @@ in
   '';
   # use all cores for building
   nix.buildCores = 0;
+  # override tmpdir for daemon
+  #systemd.services.nix-daemon.environment.TMPDIR = "/var/tmp";
 
   # package debugging
   # programs.sysdig.enable = true;