Problem: fail to build in recent nixpkgs

Solution:
- fix the expression

default.nix

@@ -69,8 +69,8 @@ buildGoApplication rec {
     -X github.com/cosmos/cosmos-sdk/version.Commit=${rev}
     -X github.com/cosmos/cosmos-sdk/version.BuildTags=${concatStringsSep "," tags}
   '';
-  postFixup = lib.optionalString stdenv.isDarwin ''
-    ${stdenv.cc.targetPrefix}install_name_tool -change "@rpath/librocksdb.8.dylib" "${rocksdb}/lib/librocksdb.dylib" $out/bin/chain-maind
+  postFixup = lib.optionalString (stdenv.isDarwin && rocksdb != null) ''
+    ${stdenv.cc.buildTools.targetPrefix}install_name_tool -change "@rpath/librocksdb.8.dylib" "${rocksdb}/lib/librocksdb.dylib" $out/bin/chain-maind
   '';
   passthru = {
     # update script use the same golang version as the project