treewide: use inputs instead of self

hosts/mail/default.nix

@@ -1,12 +1,12 @@
 {
+  inputs,
   lib,
-  self,
   ...
 }:
 
 {
   imports = [
-    self.inputs.disko.nixosModules.disko
+    inputs.disko.nixosModules.disko
     ./hardware-configuration.nix
     ./disko-config.nix
     ./mail.nix

hosts/mail/mail.nix

@@ -1,6 +1,6 @@
 {
+  inputs,
   config,
-  self,
   ...
 }:
 
@@ -20,7 +20,7 @@ let
 in
 {
   imports = [
-    self.inputs.nixos-mailserver.nixosModule
+    inputs.nixos-mailserver.nixosModule
   ];
 
   services.roundcube = {