enable lazy trees by default for testing in ci

this is so we can see what would break.

src/libexpr/include/nix/expr/eval-settings.hh

@@ -248,7 +248,7 @@ struct EvalSettings : Config
           This option can be enabled by setting `NIX_ABORT_ON_WARN=1` in the environment.
         )"};
 
-    Setting<bool> lazyTrees{this, false, "lazy-trees",
+    Setting<bool> lazyTrees{this, true, "lazy-trees",
         R"(
           If set to true, flakes and trees fetched by [`builtins.fetchTree`](@docroot@/language/builtins.md#builtins-fetchTree) are only copied to the Nix store when they're used as a dependency of a derivation. This avoids copying (potentially large) source trees unnecessarily.
         )"};