Fix image workflow: fetch extra repos from main, not primary SHA

.forgejo/workflows/image.yaml

@@ -41,7 +41,8 @@ jobs:
           for spec in ${EXTRA_REPOS}; do
             repo="${spec%@*}"
             ctx="${spec#*@}"
-            extra_ref="${REF}"
+            # Extra repos track their own main branch; primary SHA is not valid there.
+            extra_ref="main"
             wget -qO "/tmp/${ctx}.tar.gz" \
               "https://forgejo.coulomb.social/${repo}/archive/${extra_ref}.tar.gz"
             mkdir -p "/tmp/ctx-${ctx}"