Clobber Linux_unopt checkouts by default. (flutter/engine#31817)

This is to prevent licenses checks to fail when files from previous
changes are left in the cache.

Bug: https://github.com/flutter/flutter/issues/99526

engine/src/flutter/.ci.yaml

@@ -192,6 +192,7 @@ targets:
     recipe: engine/engine_unopt
     properties:
       add_recipes_cq: "true"
+      clobber: "true"
     timeout: 60
     scheduler: luci