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/libstore/optimise-store.cc

@@ -206,7 +206,9 @@ retry:
      the store itself (we don't want or need to mess with its
      permissions). */
   bool mustToggle = dirOf(path) != realStoreDir;
-  if (mustToggle) makeWritable(dirOf(path));
+  if (mustToggle) {
+    makeWritable(dirOf(path));
+  }
 
   /* When we're done, make the directory read-only again and reset
      its timestamp back to 0. */