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/nix-build/nix-build.cc

@@ -379,7 +379,9 @@ static void _main(int argc, char** argv) {
 
     buildPaths(pathsToBuild);
 
-    if (dryRun) return;
+    if (dryRun) {
+      return;
+    }
 
     // Set the environment.
     auto env = getEnv();
@@ -501,7 +503,9 @@ static void _main(int argc, char** argv) {
 
     buildPaths(pathsToBuild);
 
-    if (dryRun) return;
+    if (dryRun) {
+      return;
+    }
 
     for (auto& symlink : resultSymlinks)
       if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())