style(3p/nix): Final act in the brace-wrapping saga

This last change set was generated by a full clang-tidy run (including
compilation):

    clang-tidy -p ~/projects/nix-build/ \
      -checks=-*,readability-braces-around-statements -fix src/*/*.cc

Actually running clang-tidy requires some massaging to make it play
nice with Nix + meson, I'll be adding a wrapper or something for that soon.

third_party/nix/src/libmain/stack.cc

@@ -58,8 +58,9 @@ void detectStackOverflow() {
     throw Error("cannot allocate alternative stack");
   }
   stack.ss_flags = 0;
-  if (sigaltstack(&stack, 0) == -1)
+  if (sigaltstack(&stack, 0) == -1) {
     throw SysError("cannot set alternative stack");
+  }
 
   struct sigaction act;
   sigfillset(&act.sa_mask);