update workflow

Author: muink

.github/workflows/buildimage.yml

@@ -117,7 +117,7 @@ jobs:
           echo "$USIGN_PUBKEY" > key-build.pub
           echo "$USIGN_PUBKEY" > keys/${USIGN_ID,,}
           # core feed
-          sed -i "/src\/gz openwrt_core /{ \
+          sed -i "/src\/gz rebuild_core /{ \
             s|https://downloads.openwrt.org/releases/$VERSION|file://$GITHUB_WORKSPACE| \
           }" repositories.conf
           # fantastic feeds