Bring nix derivation show in compliance with JSON guidelines

This matches what we just did for `nix path-info`, and I hope will allow
us to avoiding any more breaking changes to this command for the
foreseeable future.

src/libstore/derivations.cc

@@ -1489,8 +1489,6 @@ adl_serializer<DerivationOutput>::from_json(const json & _json, const Experiment
     }
 }
 
-static unsigned constexpr expectedJsonVersionDerivation = 4;
-
 void adl_serializer<Derivation>::to_json(json & res, const Derivation & d)
 {
     res = nlohmann::json::object();

src/libstore/include/nix/store/derivations.hh

@@ -586,6 +586,12 @@ void writeDerivation(Sink & out, const StoreDirConfig & store, const BasicDeriva
  */
 std::string hashPlaceholder(const OutputNameView outputName);
 
+/**
+ * The expected JSON version for derivation serialization.
+ * Used by `nix derivation show` and `nix derivation add`.
+ */
+constexpr unsigned expectedJsonVersionDerivation = 4;
+
 } // namespace nix
 
 JSON_IMPL_WITH_XP_FEATURES(nix::DerivationOutput)

src/nix/derivation-show.cc

@@ -60,7 +60,11 @@ struct CmdShowDerivation : InstallablesCommand, MixPrintJSON
 
             jsonRoot[drvPath.to_string()] = store->readDerivation(drvPath);
         }
-        printJSON(jsonRoot);
+        printJSON(
+            nlohmann::json{
+                {"version", expectedJsonVersionDerivation},
+                {"derivations", std::move(jsonRoot)},
+            });
     }
 };