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/libexpr/primops.cc

@@ -1174,7 +1174,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         // hash per output.
         auto hashModulo = hashDerivationModulo(*state.store, Derivation(drv), true);
         std::visit(overloaded {
-            [&](Hash h) {
+            [&](Hash & h) {
                 for (auto & i : outputs) {
                     auto outPath = state.store->makeOutputPath(i, h, drvName);
                     drv.env[i] = state.store->printStorePath(outPath);
@@ -1186,11 +1186,11 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                         });
                 }
             },
-            [&](CaOutputHashes) {
+            [&](CaOutputHashes &) {
                 // Shouldn't happen as the toplevel derivation is not CA.
                 assert(false);
             },
-            [&](DeferredHash _) {
+            [&](DeferredHash &) {
                 for (auto & i : outputs) {
                     drv.outputs.insert_or_assign(i,
                         DerivationOutput {