[core] fix wheel timestamp check (#4488)

Previously, we were only taking the max timestamp of all the subdirectories of
the given directory. So the timestamp could be incorrect if only a file changed,
and no directory changed. This fixes the issue by looking at all directories and
files given by os.walk().

sky/backends/wheel_utils.py

@@ -153,7 +153,10 @@ def _get_latest_modification_time(path: pathlib.Path) -> float:
         if not path.exists():
             return -1.
         try:
-            return max(os.path.getmtime(root) for root, _, _ in os.walk(path))
+            return max(
+                os.path.getmtime(os.path.join(root, f))
+                for root, dirs, files in os.walk(path)
+                for f in (*dirs, *files))
         except ValueError:
             return -1.