use consumer-groups branch in build

Author: scottf

.github/workflows/build-pr.yml

@@ -31,6 +31,7 @@ jobs:
           cd $GITHUB_WORKSPACE
           git clone https://github.com/nats-io/nats-server.git
           cd nats-server
+          git checkout consumer-groups
           go get
           go build main.go
           mkdir -p ~/.local/bin