feat(tvix/eval): implement dynamic keys in recursive attrs

This wires up the new bindings setup logic to be able to thread
through & compile dynamic attributes in recursive attrs.

It seems like we don't actually need to retain the phasing of Nix
exactly, as we can use the phantom mechanism to declare all locals
without making the dynamic ones accessible.

Change-Id: Ic2d43dd8fd97d7ccd56d8c6adf2ff97274cd837a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6781
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.exp

@@ -0,0 +1 @@
+{ barbaz = 42; foobar = 42; val = 21; }

tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.nix

@@ -0,0 +1,5 @@
+rec {
+  val = 21;
+  ${"foo" + "bar"} = 42;
+  ${"bar" + "baz"} = val * 2;
+}