build: build specific enterprise version when releasing.

scripts/build/prepare-enterprise.sh

 #!/bin/bash
 
 cd ..
-git clone -b master --single-branch git@github.com:grafana/grafana-enterprise.git --depth 1
+
+
+if [ -z "$CIRCLE_TAG" ]; then
+  _target="master"
+else
+  _target="$CIRCLE_TAG"
+fi
+
+git clone -b "$_target" --single-branch git@github.com:grafana/grafana-enterprise.git --depth 1
+
 cd grafana-enterprise
 ./build.sh