fix(tvix/eval): ensure all thunks are forced in nested selects

Previously only the first one was guaranteed to be forced, but we need
to do this for all of them.

Fixes b/190

Change-Id: I76b5667dbfb2f3fde3587e7b91d268cbf32aca00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6645
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.exp

@@ -0,0 +1 @@
+42

tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.nix

@@ -0,0 +1,5 @@
+({
+  x = {
+    y = 42;
+  };
+}).x.y