std::visit by reference

I had started the trend of doing `std::visit` by value (because a type
error once mislead me into thinking that was the only form that
existed). While the optomizer in principle should be able to deal with
extra coppying or extra indirection once the lambdas inlined, sticking
with by reference is the conventional default. I hope this might even
improve performance.

src/libstore/remote-store.cc

@@ -528,13 +528,13 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
         if (repair) throw Error("repairing is not supported when building through the Nix daemon protocol < 1.25");
 
         std::visit(overloaded {
-            [&](TextHashMethod thm) -> void {
+            [&](const TextHashMethod & thm) -> void {
                 std::string s = dump.drain();
                 conn->to << wopAddTextToStore << name << s;
                 worker_proto::write(*this, conn->to, references);
                 conn.processStderr();
             },
-            [&](FixedOutputHashMethod fohm) -> void {
+            [&](const FixedOutputHashMethod & fohm) -> void {
                 conn->to
                     << wopAddToStore
                     << name
@@ -705,10 +705,10 @@ static void writeDerivedPaths(RemoteStore & store, ConnectionHandle & conn, cons
         for (auto & p : reqs) {
             auto sOrDrvPath = StorePathWithOutputs::tryFromDerivedPath(p);
             std::visit(overloaded {
-                [&](StorePathWithOutputs s) {
+                [&](const StorePathWithOutputs & s) {
                     ss.push_back(s.to_string(store));
                 },
-                [&](StorePath drvPath) {
+                [&](const StorePath & drvPath) {
                     throw Error("trying to request '%s', but daemon protocol %d.%d is too old (< 1.29) to request a derivation file",
                         store.printStorePath(drvPath),
                         GET_PROTOCOL_MAJOR(conn->daemonVersion),