fix: use 'next' branch instead of 'main' for next version

Downstream repos use a 'next' branch, not 'main'.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: waveywaves

.github/workflows/trigger-image-rebuilds.yaml

@@ -51,7 +51,7 @@ jobs:
           fi
 
           if [[ "$VERSION" == "next" ]]; then
-            BRANCH="main"
+            BRANCH="next"
           else
             BRANCH="release-v${VERSION}.x"
           fi