Fix infinite recursion when walking Abstractions

[infinisil]

This caused an infinite recursion when trying to minimise the non-idempotency when running --verify on

{
  foo
# bar
, baz # qux
}:
null

---

This work is sponsored by Antithesis ✨

[github-actions]

Nixpkgs diff

[piegamesde]

Please make the commit message more clear that this is about the walkSubprogram, because it is only used for debugging regressions and does not affect program execution otherwise