refactor(3p): fetch impermanence via niv

Change-Id: Ic6bc463304643a7f983a5f7c182f95a7706ca35b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5868
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

users/tazjin/nixos/modules/persistence.nix

@@ -3,7 +3,7 @@
 
 {
   imports = [
-    "${depot.third_party.impermanence}/nixos.nix"
+    (depot.third_party.sources.impermanence + "/nixos.nix")
   ];
 
   environment.persistence."/persist" = {

users/tazjin/nixos/zamalek/default.nix

@@ -22,7 +22,6 @@ in
     (usermod "physical.nix")
     (usermod "zerotier.nix")
 
-    (depot.third_party.impermanence + "/nixos.nix")
     (pkgs.home-manager.src + "/nixos")
   ] ++ lib.optional (builtins.pathExists ./local-config.nix) ./local-config.nix;