feat(tazjin/nixos): set up yggdrasil on all physical machines

Change-Id: I1d722c6d2e6192c445e77cb9b186a5320c2a60ec
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12485
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

users/tazjin/nixos/modules/physical.nix

@@ -102,6 +102,13 @@ in
     # run manually patchelfed binaries
     environment.stub-ld.enable = false;
 
+    # Enable yggdrasil network.
+    services.yggdrasil = {
+      enable = true;
+      persistentKeys = true;
+      settings.IfName = "ygg0";
+    };
+
     programs = {
       fish.enable = true;
       mosh.enable = true;