fix: invalid cwd when running bytecode

[DonIsaac]

What does this PR do?

Fixes a bug where dirname_fd was always invalid when running compiled bytecode.

How did you verify your code works?

There is already a test for this; it was never caught because the invalid fd assertion only ran in debug builds.

[robobun]

^{Updated 5:14 PM PT - Apr 20th, 2025}

❌ @DonIsaac, your commit 82dfec9 has 1 failures in Build #15235:

• test/js/third_party/body-parser/express-memory-leak.test.ts - 1 failing on 🍎 14 aarch64

---

🧪   try this PR locally:

bunx bun-pr 19148