Add tx-cost to nix packages (#1662)

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---

<!-- Consider each and tick it off one way or the other -->
* [x] CHANGELOG updated or not needed
* [x] Documentation updated or not needed
* [x] Haddocks updated or not needed
* [x] No new TODOs introduced or explained herafter

nix/hydra/packages.nix

@@ -105,6 +105,9 @@ rec {
   hydra-explorer-web =
     import "${self}/hydra-explorer/web/hydra-explorer.nix" { inherit pkgs; };
 
+  tx-cost =
+    nativePkgs.hydra-node.components.benchmarks.tx-cost;
+
   hydra-tui =
     embedRevision
       nativePkgs.hydra-tui.components.exes.hydra-tui