nix store copy-sigs: Use http-connections setting to control parallelism

Previously it used the `ThreadPool` default,
i.e. `std:🧵:hardware_concurrency()`. But copying signatures is
not primarily CPU-bound so it makes more sense to use the
`http-connections` setting (since we're typically copying from/to a
binary cache).

src/nix/sigs.cc

@@ -3,6 +3,7 @@
 #include "nix/main/shared.hh"
 #include "nix/store/store-open.hh"
 #include "nix/util/thread-pool.hh"
+#include "nix/store/filetransfer.hh"
 
 #include <atomic>
 
@@ -38,7 +39,7 @@ struct CmdCopySigs : StorePathsCommand
         for (auto & s : substituterUris)
             substituters.push_back(openStore(s));
 
-        ThreadPool pool;
+        ThreadPool pool{fileTransferSettings.httpConnections};
 
         std::atomic<size_t> added{0};