fix(tvix/eval): wrap asserts in a thunk

As the new test case demonstrates, asserts need to be evaluated lazily.

Change-Id: I808046722c5a504e9497855ca5026d255c7a4c34
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6494
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.exp

@@ -0,0 +1 @@
+12

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

@@ -0,0 +1,8 @@
+assert true;
+
+let
+  x = assert false; 13;
+  y = 12;
+in
+
+{ inherit x y; }.y