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/libutil/lazy.hh

@@ -35,7 +35,9 @@ class Lazy {
         ex = std::current_exception();
       }
     });
-    if (ex) std::rethrow_exception(ex);
+    if (ex) {
+      std::rethrow_exception(ex);
+    }
     return value;
   }
 };