refactor(3p/nix): Apply clang-tidy's performance-* fixes r/789

This applies the performance fixes listed here:

https://clang.llvm.org/extra/clang-tidy/checks/list.html

1 files changed, 2 insertions, 2 deletions

diff --git a/third_party/nix/src/nix/verify.cc b/third_party/nix/src/nix/verify.cc
index 63dd181e07..a773b3a2c8 100644
--- a/third_party/nix/src/nix/verify.cc
+++ b/third_party/nix/src/nix/verify.cc
@@ -102,8 +102,8 @@ struct CmdVerify : StorePathsCommand {
             size_t actualSigsNeeded = std::max(sigsNeeded, (size_t)1);
             size_t validSigs = 0;
 
-            auto doSigs = [&](StringSet sigs) {
-              for (auto sig : sigs) {
+            auto doSigs = [&](const StringSet& sigs) {
+              for (const auto& sig : sigs) {
                 if (sigsSeen.count(sig) != 0u) {
                   continue;
                 }