try-runtime: don't require --block-at in snapshot

[kianenigma]

in essence,

/// The block hash at which to read state. This is required for execute-block, offchain-worker,
/// or any command that used the live subcommand.
#[structopt(
	short,
	long,
	multiple = false,
	parse(try_from_str = parse::hash),
	required_ifs(
		&[("command", "offchain-worker"), ("command", "execute-block"), ("subcommand", "live")]
	)
)]
block_at: String,

the required_ifs is not really doing what it is supposed to do. I still can't run on-runtime-upgrade <...> snap <...> without giving a (seemingly unused block-at).

cc @emostov