test(tvix/eval): add simple nested key tests in let & rec attrs

Change-Id: Icdc41ae383cd7cd1531c65c97d955bc59a56489a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6809
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.exp

@@ -0,0 +1 @@
+{ a = { b = 42; }; }