Use buffered channel for signal.Notify

[Currently an unbuffered channel is used for os signals](https://github.com/snowplow/snowbridge/blob/c8c94f8a13367260f9a1a280cdd5d86098f66521/cmd/cli/cli.go#L123). [According to the docs](https://pkg.go.dev/os/signal#Notify) a buffered channel must be used.