infra: image: fixes image_tag

.gitea/workflows/gentoo-utils.yml

@@ -26,7 +26,8 @@ jobs:
 
           echo "branch_name=$branch_name" >> $GITEA_OUTPUT
           # slugify
-          echo "image_tag=$(echo "$image_tag" | sed -E 's/[^a-zA-Z0-9]/-/g')" >> $GITEA_OUTPUT
+          image_tag="$(echo "$image_tag" | sed -E 's/[^a-zA-Z0-9]/-/g')"
+          echo "image_tag=$image_tag" >> $GITEA_OUTPUT
 
           if ! git diff ${{ gitea.event.before }} ${{ gitea.sha }} --no-patch --exit-code .docker; then
             build_image=true