Enable clap env feature

The CLI reads SHANTY_DATABASE_URL via #[arg(env)], which needs the
feature. It was previously only enabled through sea-orm-cli.

Cargo.toml

@@ -9,7 +9,7 @@ repository = "ssh://connor@git.rcjohnstone.com:2222/Shanty/dl.git"
 [dependencies]
 shanty-db = { path = "../shanty-db" }
 sea-orm = { version = "1", features = ["sqlx-sqlite", "runtime-tokio-native-tls"] }
-clap = { version = "4", features = ["derive"] }
+clap = { version = "4", features = ["derive", "env"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 thiserror = "2"