fixed matrix reference

Author: mrinc

.github/workflows/buildAndPub.yml

@@ -102,7 +102,7 @@ jobs:
         run: |
           # Only keep the highest version of the last 3 major versions for devcontainers
           jq '[. | sort_by(.major) | reverse | group_by(.major) | map(.[0]) | sort_by(.major) | reverse | .[:3]]' config.json > devcontainer_matrix.json
-          echo "matrix=$(jq -c '{"tag": .}' devcontainer_matrix.json)" >> $GITHUB_OUTPUT
+          echo "devcontainer_matrix=$(jq -c '{"tag": .}' devcontainer_matrix.json)" >> $GITHUB_OUTPUT
 
   build:
     needs: prepare-matrix