fix: ignore symlinks in fsync-store-paths (backport #12103)

[mergify]

Motivation

When fsync-store-paths is enabled, during nix copy symlink store entries are treated as directory, which is not correct.

Context

Fixes #12099

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This is an automatic backport of pull request #12103 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4a91e62 has failed:

On branch mergify/bp/2.24-maintenance/pr-12103
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You are currently cherry-picking commit 4a91e627a.
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Unmerged paths:
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	both modified:   src/libutil/file-system.cc

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