fix throwing output paths out of sandbox paths (backport #13356)

[mergify]

Motivation

Building paths that are in sandbox-paths broke in some recent changes. This restores that ability.

Context

Correct me if I'm wrong but it seems obvious that erasing any output paths from pathsInChroot needs to happen after getPathsInSandbox(), not before.

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This is an automatic backport of pull request #13356 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0f6cb33 has failed:

On branch mergify/bp/2.28-maintenance/pr-13356
Your branch is up to date with 'origin/2.28-maintenance'.

You are currently cherry-picking commit 0f6cb3376.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   src/libstore/unix/build/linux-derivation-builder.cc

no changes added to commit (use "git add" and/or "git commit -a")

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