clanServices/wifi: fix for multiple instances

Without this, `nix build .#checks.x86_64-linux.wifi` fails with:
```
error: The option `nodes.first.systemd.services.NetworkManager-setup-secrets.serviceConfig.ExecStart' has conflicting definition values:
- In `/nix/store/x0…45-source/clanServices/wifi/default.nix, via option mappedServices."self-@clan/wifi".roles.default.perInstance, via option nixosModule': <derivation wifi-secrets>
- In `/nix/store/x0…45-source/clanServices/wifi/default.nix, via option mappedServices."self-@clan/wifi".roles.default.perInstance, via option nixosModule': <derivation wifi-secrets>
Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.
```

clanServices/wifi/default.nix

@@ -39,7 +39,7 @@ in
     };
 
     perInstance =
-      { settings, ... }:
+      { instanceName, settings, ... }:
       {
         nixosModule =
           { pkgs, config, ... }:
@@ -86,7 +86,7 @@ in
 
             # service to generate the environment file containing all secrets, as
             #   expected by the nixos NetworkManager-ensure-profile service
-            systemd.services.NetworkManager-setup-secrets = {
+            systemd.services."NetworkManager-setup-secrets-${instanceName}" = {
               description = "Generate wifi secrets for NetworkManager";
               requiredBy = [ "NetworkManager-ensure-profiles.service" ];
               partOf = [ "NetworkManager-ensure-profiles.service" ];