modules: services: rename 'nix-cache'

I'm about to modify the module to use a different cache implementation,
so this name is more appropriate.

hosts/nixos/porthos/services.nix

@@ -93,7 +93,7 @@ in
       enable = true;
       passwordFile = secrets."nextcloud/password".path;
     };
-    nix-serve = {
+    nix-cache = {
       enable = true;
       secretKeyFile = secrets."nix-serve/cache-key".path;
     };

modules/services/default.nix

@@ -20,7 +20,7 @@
     ./navidrome
     ./nextcloud
     ./nginx
-    ./nix-serve
+    ./nix-cache
     ./paperless
     ./pirate
     ./podgrab

modules/services/nix-cache/default.nix

@@ -1,11 +1,11 @@
 # Binary cache through nix-serve
 { config, lib, pkgs, ... }:
 let
-  cfg = config.my.services.nix-serve;
+  cfg = config.my.services.nix-cache;
 in
 {
-  options.my.services.nix-serve = with lib; {
-    enable = mkEnableOption "nix-serve binary cache";
+  options.my.services.nix-cache = with lib; {
+    enable = mkEnableOption "nix binary cache";
 
     port = mkOption {
       type = types.port;
@@ -16,7 +16,7 @@ in
 
     secretKeyFile = mkOption {
       type = types.str;
-      example = "/run/secrets/nix-serve";
+      example = "/run/secrets/nix-cache";
       description = "Secret signing key for the cache";
     };