Raise minimum version for testing cxxbridge-cmd to Rust 1.70

Required by clap 4.4.0.

    error: package `clap v4.4.0` cannot be built because it requires rustc
    1.70.0 or newer, while the currently active rustc version is 1.64.0

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
           - rust: beta
           - rust: stable
           - rust: 1.60.0
-          - rust: 1.64.0
+          - rust: 1.70.0
           - name: Cargo on macOS
             rust: nightly
             os: macos