github workflows: replace deprecated set-env

.github/workflows/joystream-node-docker.yml

@@ -55,7 +55,7 @@ jobs:
             docker save --output joystream-node-docker-image.tar joystream/node
             gzip joystream-node-docker-image.tar
             cp joystream-node-docker-image.tar.gz ~/docker-images/
-            echo "::set-env name=NEW_BUILD::true"
+            echo "NEW_BUILD=1" >> $GITHUB_ENV
           fi
 
       - name: Save joystream/node image to Artifacts