Fix the Kotlin and Dart release jobs
Both broke on the 1.2.3 tag for reasons unrelated to the release itself. Kotlin: dtolnay/rust-toolchain@stable installed stable and added the four Android targets to it, but bare cargo in this repo resolves to the 1.94.0 pin in rust-toolchain.toml, which has no Android std — so building the .so files died on "can't find crate for `core`". ci.yml and the Swift and Rust release workflows already read the channel out of rust-toolchain.toml and feed it to the action; this does the same. Dart: flutter pub get on 3.44+ rewrites analysis_options.yaml for the package and the example to exclude build and platform directories, so pub publish --dry-run saw two modified checked-in files, warned, and exited 65. The publish job already restores the tree after pub get for the same reason with the plugin registrants; validate now does too. Co-Authored-By: siGit Code v1.5.6 <noreply@sigit.si>