move_unpacked_nix: Read source directory all at once

[edolstra]

Description

Otherwise, at least on XFS, it's possible that we encounter store paths twice.

Should fix the failure seen in https://github.com/DeterminateSystems/nix-installer/actions/runs/13162433419/job/36734918342.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install