Adjust file check

action.yml

@@ -166,7 +166,7 @@ runs:
             # Compress Armbian image file
             pigz -qf *.img || gzip -qf *.img
             # Generate a sha256sum verification file for each Armbian image file
-            for file in *; do [[ ! -d "${file}" ]] && sha256sum "${file}" >"${file}.sha"; done
+            for file in *; do [[ -f "${file}" ]] && sha256sum "${file}" >"${file}.sha" 2>/dev/null; done
             rm -f *.sha.sha 2>/dev/null
             sync && sleep 3