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

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

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

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

third_party/nix/src/nix-channel/nix-channel.cc

@@ -148,7 +148,9 @@ static void update(const StringSet& channelNames) {
   Strings envArgs{"--profile", profile,
                   "--file",    "<nix/unpack-channel.nix>",
                   "--install", "--from-expression"};
-  for (auto& expr : exprs) envArgs.push_back(std::move(expr));
+  for (auto& expr : exprs) {
+    envArgs.push_back(std::move(expr));
+  }
   envArgs.push_back("--quiet");
   runProgram(settings.nixBinDir + "/nix-env", false, envArgs);