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/libexpr/eval-inline.hh

@@ -81,7 +81,9 @@ inline void* allocBytes(size_t n) {
 #else
   p = calloc(n, 1);
 #endif
-  if (!p) throw std::bad_alloc();
+  if (!p) {
+    throw std::bad_alloc();
+  }
   return p;
 }