[CI] Add sleep to buildbot startup script

buildbot-worker start does start the buildbot worker, but in a background process.
This means that the container then immediately exists, also killing the worker. Add
a sleep at the end to prevent termination.

Author: boomanaiden154

premerge/buildbot/startup.sh

@@ -18,3 +18,5 @@ echo "Google LLVM Premerge Infra Rotation <llvm-presubmit-infra@google.com>" \
 } > /worker/info/host
 
 buildbot-worker start /worker
+
+sleep 31536000000