[Heartbeat] Use stack_release label for synthetics npm install (#33362)

Fixes #31732

Always use the `stack_release` label for `npm i`

(cherry picked from commit f3302cd)

dev-tools/packaging/templates/docker/Dockerfile.tmpl

@@ -151,7 +151,7 @@ RUN cd /usr/share/heartbeat/.node \
 RUN chown -R {{ .user }} $NODE_PATH
 USER {{ .user }}
 # If this fails dump the NPM logs
-RUN npm i -g --loglevel verbose -f @elastic/synthetics || sh -c 'tail -n +1 /root/.npm/_logs/* && exit 1'
+RUN npm i -g --loglevel verbose -f @elastic/synthetics@stack_release || sh -c 'tail -n +1 /root/.npm/_logs/* && exit 1'
 RUN chmod ug+rwX -R $NODE_PATH 
 USER root