nixos/tmproot: Persist Nix cache

nixos/modules/tmproot.nix

@@ -143,6 +143,8 @@ in
             # Also the script runs before impermanence does.
             "/var/lib/nixos"
             "/var/lib/systemd"
+
+            { directory = "/root/.cache/nix"; mode = "0700"; }
           ];
           files = [
             "/etc/machine-id"

nixos/modules/user.nix

@@ -76,6 +76,8 @@ in
               # Persist all of fish; it's not easy to persist just the history fish won't let you move it to a different
               # directory. Also it does some funny stuff and can't really be a symlink it seems.
               "/home/${user'.name}/.local/share/fish"
+
+              "/home/${user'.name}/.cache/nix"
             ];
           };
         };