refactor(treewide): reserve vector capacity when final size is known

In these trivial cases the final vector size (or lower bound on the size) is known,
so we can avoid some vector reallocations. This is not very important, but is just
good practice and general hygiene.

src/libexpr/primops.cc

@@ -91,6 +91,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context, StorePathS
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
+    buildReqs.reserve(drvs.size());
     for (auto & d : drvs) buildReqs.emplace_back(DerivedPath { d });
     buildStore->buildPaths(buildReqs, bmNormal, store);