style(3p/nix): Add braces around single-line conditionals

These were not caught by the previous clang-tidy invocation, but were
instead sorted out using amber[0] as such:

    ambr --regex 'if (\(.+\))\s([a-z].*;)' 'if $1 { $2 }'

[0]: https://github.com/dalance/amber

third_party/nix/src/libstore/ssh-store.cc

@@ -77,7 +77,9 @@ ref<RemoteStore::Connection> SSHStore::openConnection() {
 static RegisterStoreImplementation regStore([](const std::string& uri,
                                                const Store::Params& params)
                                                 -> std::shared_ptr<Store> {
-  if (std::string(uri, 0, uriScheme.size()) != uriScheme) return 0;
+  if (std::string(uri, 0, uriScheme.size()) != uriScheme) {
+    return 0;
+  }
   return std::make_shared<SSHStore>(std::string(uri, uriScheme.size()), params);
 });