checks backup: limit to linux systems

checks/backups/flake-module.nix

@@ -64,7 +64,8 @@ in
       };
   };
   perSystem = { nodes, pkgs, ... }: {
-    checks.test-backups =
+    checks = pkgs.lib.mkIf (pkgs.stdenv.isLinux) {
+      test-backups =
         (import ../lib/test-base.nix)
           {
             name = "test-backups";
@@ -122,4 +123,5 @@ in
           }
           { inherit pkgs self; };
     };
+  };
 }