build(deps): update tough requirement from 0.18 to 0.19

Updates the requirements on [tough](https://github.com/awslabs/tough) to permit the latest version.
- [Release notes](https://github.com/awslabs/tough/releases)
- [Commits](awslabs/tough@tough-v0.18.0...tough-v0.19.0)

---
updated-dependencies:
- dependency-name: tough
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -142,7 +142,7 @@ sigstore_protobuf_specs = { version = "0.3", optional = true }
 thiserror = "1.0"
 tokio = { version = "1", features = ["rt"] }
 tokio-util = { version = "0.7", features = ["io-util"] }
-tough = { version = "0.18", features = ["http"], optional = true }
+tough = { version = "0.19", features = ["http"], optional = true }
 tracing = "0.1"
 url = "2.5"
 x509-cert = { version = "0.2", features = ["builder", "pem", "std", "sct"] }