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/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!"; }