fix(tvix/eval): account for attrset temporaries during construction

The temporaries left on the stack as operands to `OpAttrs` must be
accounted for in the locals array in order for operations within them
to receive correct slots.

Some test cases that were previously broken have been added.

Change-Id: Ib52b629bbdf7931f63fd45a45af1073022da923c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6468
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.exp

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

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

@@ -0,0 +1,7 @@
+# Creates a `with` across multiple thunk boundaries.
+
+let
+  set = {
+    a = with { b = 42; }; b;
+  };
+in set.a

tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.exp

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

tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.nix

@@ -0,0 +1,12 @@
+# Tests correct tracking of stack indices within construction of an
+# attribute set. Dynamic keys can be any expression, so something that
+# is extremely sensitive to stack offsets (like `with`) can be tricky.
+
+let
+  set1 = { key = "b"; };
+  set2 = {
+    a = 20;
+    ${with set1; key} = 20;
+    ${with { key = "c"; }; key} = 2;
+  };
+in set2.a + set2.b + set2.c