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/local-store.cc

@@ -681,7 +681,7 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
     std::optional<Hash> h;
     for (auto & i : drv.outputs) {
         std::visit(overloaded {
-            [&](DerivationOutputInputAddressed doia) {
+            [&](const DerivationOutputInputAddressed & doia) {
                 if (!h) {
                     // somewhat expensive so we do lazily
                     auto temp = hashDerivationModulo(*this, drv, true);
@@ -693,14 +693,14 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
                         printStorePath(drvPath), printStorePath(doia.path), printStorePath(recomputed));
                 envHasRightPath(doia.path, i.first);
             },
-            [&](DerivationOutputCAFixed dof) {
+            [&](const DerivationOutputCAFixed & dof) {
                 StorePath path = makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
                 envHasRightPath(path, i.first);
             },
-            [&](DerivationOutputCAFloating _) {
+            [&](const DerivationOutputCAFloating &) {
                 /* Nothing to check */
             },
-            [&](DerivationOutputDeferred) {
+            [&](const DerivationOutputDeferred &) {
             },
         }, i.second.output);
     }