Update buildx setup instructions to use `install: true`

Currently, `install` defaults to false.

Setting `install` to true makes `docker build` use the new builder.

content/build/hydrobuild.md

@@ -236,6 +236,7 @@ jobs:
           version: "lab:latest"
           driver: cloud
           endpoint: "<ORG>/default"
+          install: true
       - name: Build and push
         uses: docker/build-push-action@v5
         with: