refactor(users/tazjin): Extract non-home persistence config

This will be reused between machines.

Change-Id: Ice7835ce7e63b00284e046606309e882412cda26
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5547
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>

users/tazjin/nixos/modules/zerotier.nix

@@ -0,0 +1,12 @@
+# Configuration for my Zerotier network.
+
+{
+  environment.persistence."/persist".directories = [
+    "/var/lib/zerotier-one"
+  ];
+
+  services.zerotierone.enable = true;
+  services.zerotierone.joinNetworks = [
+    "35c192ce9bd4c8c7"
+  ];
+}