nixos/wireguard-networkd: fix systemd credentials not working with multiple keys per interface

This patch fixes systemd credential loading for wireguard
with networkd not working when specifying more than one
credential per interface by properly flattening the resulting
multidimensional list instead of falling back on string concat
with a space.

Resolves #365402

nixos/modules/services/networking/wireguard-networkd.nix

@@ -14,7 +14,12 @@ let
     mapAttrsToList
     nameValuePair
     ;
-  inherit (lib.lists) concatMap concatLists filter;
+  inherit (lib.lists)
+    concatMap
+    concatLists
+    filter
+    flatten
+    ;
   inherit (lib.modules) mkIf;
   inherit (lib.options) literalExpression mkOption;
   inherit (lib.strings) hasInfix;
@@ -215,7 +220,9 @@ in
 
     systemd.timers = mapAttrs' generateRefreshTimer refreshEnabledInterfaces;
     systemd.services = (mapAttrs' generateRefreshService refreshEnabledInterfaces) // {
-      systemd-networkd.serviceConfig.LoadCredential = mapAttrsToList interfaceCredentials cfg.interfaces;
+      systemd-networkd.serviceConfig.LoadCredential = flatten (
+        mapAttrsToList interfaceCredentials cfg.interfaces
+      );
     };
   };
 }