Set the checksum directory explicitly

This matches the default, but makes the intent more clear and is
resilient to changes in behavior.

Author: elasticdog

.github/workflows/draft-release-flutter.yml

@@ -272,7 +272,7 @@ jobs:
             common
 
       - name: Generate combined checksums
-        run: create-checksums
+        run: create-checksums --dir dist
 
       - name: Upload release assets
         env: