Fix `stack sdist`

Description

The same way as it was done in 08704a90. Refer to that commit for details.

Also see https://github.com/sol/hpack/issues/303

Related issue(s)

None

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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