crypto: Fix scope of `ssh-key` dependency — heartwood

radicle-crypto is the only crate that actually depends on ssh-key. I overlooked that previously and added it as a workspace dependency. Now I am limiting its scope again.

 signature = "2.2"
 snapbox = "0.4.3"
 sqlite = "0.32.0"
 ssh-key = { version = "0.6.3", default-features = false }
 tempfile = "3.3.0"
 thiserror = "1.0"
 zeroize = "1.5.7"

 serde = { workspace = true, features = ["derive"] }
 signature = { workspace = true }
 sqlite = { workspace = true, features = ["bundled"], optional = true }
 ssh-key = { workspace = true, features = ["std", "encryption", "getrandom"], optional = true }
 ssh-key = { version = "0.6.3", default-features = false, features = ["std", "encryption", "getrandom"], optional = true }
 thiserror = { workspace = true }
 zeroize = { workspace = true }