fix(tvix/eval): wrap dynamic resolution in an extra thunk

Without this change it was possible to cause situations (see the new
test) in which a `with`-namespace was forced prematurely.

Change-Id: I879ea7763b43edc693feace2c73c890d426fafd3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7031
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>

tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.exp

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

tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.nix

@@ -0,0 +1,5 @@
+# The 'namespace' of a with should only be evaluated if an identifier
+# from it is actually accessed.
+
+with (abort "should not be evaluated");
+let a = dynamic; in 42

tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.nix

@@ -1,6 +1,6 @@
 # The 'namespace' of a with should only be evaluated if an identifier
 # from it is actually accessed.
 
-with (builtins.throw "should not occur");
+with (abort "should not be evaluated");
 
 42