Mark fetchTree as unstable again

As discussed in our last meeting, we need a bit more time, but we are
"time boxing" the work left to do to ensure there is not unbounded
delay.

Rather than putting it back underneath `flakes`, though, put it
underneath its own `fetch-tree` experimental feature (which `flakes`
includes/implies). This signals our commitment to the plan to stabilize
it first without waiting to go through the rest of Flakes, and also will
give users a "release candidate" when we get closer to stabilization.

This reverts commit 4112dd1fc9.

src/libexpr/primops/fetchTree.cc

@@ -228,6 +228,7 @@ static RegisterPrimOp primop_fetchTree({
           ```
     )",
     .fun = prim_fetchTree,
+    .experimentalFeature = Xp::FetchTree,
 });
 
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,