Rather than randomly generating a new host key every time we run the
server, load the host's secret key from a file on disk at startup, so
that clients don't have to disable host key verification to connect
every time we restart.
Change-Id: I4d283bc919f4825789f686a98c174a71929087a6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3819
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>