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/store-api.hh

@@ -747,7 +747,9 @@ struct RegisterStoreImplementation {
   static Implementations* implementations;
 
   RegisterStoreImplementation(OpenStore fun) {
-    if (!implementations) implementations = new Implementations;
+    if (!implementations) {
+      implementations = new Implementations;
+    }
     implementations->push_back(fun);
   }
 };