build(nix): prevent deprecation warning on evaluation (#3006)

Evaluating the overlay produces the following evaluation warning:
`'system' has been renamed to/replaced by 'stdenv.hostPlatform.system'`

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flake.nix

@@ -68,7 +68,7 @@
       })
     // {
       overlays.default = final: prev: {
-        inherit (self.packages.${final.system}) atuin;
+        inherit (self.packages.${final.stdenv.hostPlatform.system}) atuin;
       };
     };
 }