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/http-binary-cache-store.cc

@@ -77,8 +77,9 @@ class HttpBinaryCacheStore : public BinaryCacheStore {
     } catch (DownloadError& e) {
       /* S3 buckets return 403 if a file doesn't exist and the
          bucket is unlistable, so treat 403 as 404. */
-      if (e.error == Downloader::NotFound || e.error == Downloader::Forbidden)
+      if (e.error == Downloader::NotFound || e.error == Downloader::Forbidden) {
         return false;
+      }
       maybeDisable();
       throw;
     }