Use agenix for the buildkite ssh key and agent token on mugwump, instead
of storing stuff in /etc/secrets
Change-Id: I56951587b949fc0854e56f5c4e33b601e9cd964e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5027
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
