style(3p/nix): Enforce braces around loops and conditionals

This change was generated with:

  fd -e cc -e hh | xargs -I{} clang-tidy {} -p ~/projects/nix-build/ \
    --checks='-*,readability-braces-around-statements' --fix \
    -fix-errors

Some manual fixes were applied because some convoluted unbraced
statements couldn't be untangled by clang-tidy.

This commit still includes invalid files, but I decided to clean them
up in a subsequent commit so that it becomes more obvious where
clang-tidy failed. Maybe this will allow for a bug-report to
clang-tidy.

third_party/nix/src/libutil/affinity.cc

@@ -36,7 +36,9 @@ void setAffinityTo(int cpu) {
 int lockToCurrentCPU() {
 #if __linux__
   int cpu = sched_getcpu();
-  if (cpu != -1) setAffinityTo(cpu);
+  if (cpu != -1) {
+    setAffinityTo(cpu);
+  }
   return cpu;
 #else
   return -1;