copyPathToStore(): Preserve symlinks

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.

tests/functional/symlinks.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+# Check that when we have a derivation attribute that refers to a
+# symlink, we copy the symlink, not its target.
+# shellcheck disable=SC2016
+nix build --impure --no-link --expr '
+  with import ./config.nix;
+
+  mkDerivation {
+    name = "simple";
+    builder = builtins.toFile "builder.sh" "[[ -L \"$symlink\" ]]; mkdir $out";
+    symlink = ./lang/symlink-resolution/foo/overlays;
+  }
+'