CI: skip dirty patch check to avoid GH action bug

https://github.com/openwrt/gh-action-sdk/blob/9c3ea37/entrypoint.sh#L124
invokes `git -C $dir diff` with a non-existent $dir, which makes it
prints its --help message and exit with 128

.github/workflows/openwrt-ipkg.yml

@@ -26,6 +26,7 @@ jobs:
           FEED_DIR: "$GITHUB_WORKSPACE/openwrt/"
           PACKAGES: snapcast snapclient snapserver
           V: sc  # verbosity, see https://openwrt.org/docs/techref/buildroot#warnings_errors_and_tracing
+          NO_REFRESH_CHECK: 1  # avoid bug in git diff invocation, checking out from CI we can't have dirty patches anyway
 
       - name: Store packages
         uses: actions/upload-artifact@v4