treewide: dedup global persistence configuration

nixos/hosts/flandre-m5p/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/kanako-hkg0/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/marisa-7d76/disko-fs.nix

@@ -1,7 +0,0 @@
-{ ... }:
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/marisa-a7s/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/reisen-fra0/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/reisen-nrt0/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/reisen-sea0/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/reisen-sin0/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/hosts/suwako-vie0/disko-fs.nix

@@ -1,6 +0,0 @@
-{
-  environment.globalPersistence = {
-    enable = true;
-    root = "/persist";
-  };
-}

nixos/profiles/system/global-persistence.nix

@@ -1,6 +1,17 @@
-{ ... }:
+{ config, ... }:
 {
+  assertions = [
+    {
+      assertion = config.fileSystems ? "/persist";
+      message = ''
+        `config.fileSystems."/persist"` must be set.
+      '';
+    }
+  ];
+
   environment.globalPersistence = {
+    enable = true;
+    root = "/persist";
     directories = [
       "/var/cache"
       "/var/lib"