fix(tvix/eval): emit correct count in OpAttrPath

Not sure how exactly this snuck in, but it caused some subtle
breakages in deeply nested attribute sets.

Change-Id: I8049ce912405d3750031f79cc8d86ff1c3c02c2b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6208
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/compiler.rs

@@ -393,7 +393,7 @@ impl Compiler {
             // otherwise we need to emit an instruction to construct
             // the attribute path.
             if key_count > 1 {
-                self.chunk.push_op(OpCode::OpAttrPath(2));
+                self.chunk.push_op(OpCode::OpAttrPath(key_count));
             }
 
             // The value is just compiled as normal so that its

tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.exp

@@ -0,0 +1 @@
+{ a = { b = { c = { d = { e = { f = { g = "deep!"; }; }; }; }; }; }; }

tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.nix

@@ -0,0 +1 @@
+{ a.b.c.d.e.f.g = "deep!"; }

tvix/eval/src/value/attrs.rs

@@ -253,7 +253,7 @@ impl NixAttrs {
                     continue;
                 }
 
-                other => panic!("unexpected attribute key type: {}", other.type_of()),
+                other => panic!("unexpected attribute key: {} :: {}", other, other.type_of()),
             }
         }