linux-builder: remove /nix/store external directory when disabled

When /nix/store internal directories get renamed, they just don't get
into the next version of your system closure and are thus no problem to
rename. But state in the system is a problem, as there is no process to
remov eit. Thus we need to do it ourselves.

modules/nix/linux-builder.nix

@@ -159,7 +159,15 @@ in
     '';
   };
 
-  config = mkIf cfg.enable {
+  config = mkMerge [
+    # get rid of /nix/store external directory when disabled
+    (mkIf (!cfg.enable) {
+      system.activationScripts.preActivation.text = ''
+        rm -rf ${cfg.workingDirectory}
+      '';
+    })
+    # normal config when enabled
+    (mkIf cfg.enable {
       assertions = [
         {
           assertion = config.nix.enable;
@@ -223,5 +231,6 @@ in
       }];
 
       nix.settings.builders-use-substitutes = true;
-  };
+    })
+  ];
 }