Derivation::toJSON: fix bug!

When I moved this code from the binary to libnixstore #7863, I forgot to
display the environment variables!

src/libstore/derivations.cc

@@ -945,6 +945,7 @@ nlohmann::json Derivation::toJSON(const Store & store) const
     res["system"] = platform;
     res["builder"] = builder;
     res["args"] = args;
+    res["env"] = env;
 
     return res;
 }

src/libstore/tests/derivation.cc

@@ -85,6 +85,9 @@ TEST_JSON(Derivation, impure,
         "bar",
         "baz"
       ],
+      "env": {
+        "BIG_BAD": "WOLF"
+      },
       "outputs": {}
     })",
     ({
@@ -107,6 +110,12 @@ TEST_JSON(Derivation, impure,
             "bar",
             "baz",
         };
+        drv.env = {
+            {
+                "BIG_BAD",
+                "WOLF",
+            },
+        };
         drv;
     }))