local-store: fix infinite loop on Windows

Also switch to std::filesystem.

src/libstore/local-store.cc

@@ -212,16 +212,15 @@ LocalStore::LocalStore(
 
     /* Ensure that the store and its parents are not symlinks. */
     if (!settings.allowSymlinkedStore) {
-        Path path = realStoreDir;
-        struct stat st;
-        while (path != "/") {
-            st = lstat(path);
-            if (S_ISLNK(st.st_mode))
+        std::filesystem::path path = realStoreDir.get();
+        std::filesystem::path root = path.root_path();
+        while (path != root) {
+            if (std::filesystem::is_symlink(path))
                 throw Error(
                         "the path '%1%' is a symlink; "
                         "this is not allowed for the Nix store and its parent directories",
                         path);
-            path = dirOf(path);
+            path = path.parent_path();
         }
     }