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/libmain/common-args.cc

@@ -32,7 +32,9 @@ MixCommonArgs::MixCommonArgs(const string& programName)
   globalConfig.convertToArgs(*this, cat);
 
   // Backward compatibility hack: nix-env already had a --system flag.
-  if (programName == "nix-env") longFlags.erase("system");
+  if (programName == "nix-env") {
+    longFlags.erase("system");
+  }
 
   hiddenCategories.insert(cat);
 }