libstore: deserialize findRoots data as vector-of-tuples

a size_t followed by as many pairs of things is exactly the format of a
vector of two-element tuples. it would also be the format of a map, but
Roots is a map of sets. rather than adding a serialization format fixed
to this map type (or some wrapper) we can deserialize the response as a
vector and convert it to the map-of-sets later as this is not run much.

Change-Id: I3950c0f7cc59661576170ace10b25a6f8af1464b

lix/libstore/remote-store.cc

@@ -583,11 +583,11 @@ try {
     auto conn(TRY_AWAIT(getConnection()));
     conn->to << WorkerProto::Op::FindRoots;
     conn.processStderr();
-    size_t count = readNum<size_t>(conn->from);
+
+    auto roots =
+        WorkerProto::Serialise<std::vector<std::tuple<std::string, StorePath>>>::read(*conn);
     Roots result;
-    while (count--) {
-        Path link = readString(conn->from);
-        auto target = parseStorePath(readString(conn->from));
+    for (auto & [link, target] : roots) {
         result[std::move(target)].emplace(link);
     }
     co_return result;