copyPathToStore(): Preserve symlinks (backport #12448)

[mergify]

Motivation

E.g. in a derivation attribute foo = ./bar, if ./bar is a symlink, we should copy the symlink to the store, not its target. This restores the behaviour of Nix <= 2.19.

Context

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This is an automatic backport of pull request #12448 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 26b87e7 has failed:

On branch mergify/bp/2.24-maintenance/pr-12448
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 26b87e78b.
  (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)

Changes to be committed:
	modified:   src/libexpr/eval.cc
	modified:   tests/functional/simple.sh
	new file:   tests/functional/symlinks.sh

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   tests/functional/meson.build

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally