Fixes to Rust setup in CI

[cmnrd]

This PR solves two issues:

1. It adds a workaround for a problem in the action that we are using for setting up rust (https://github.com/ATiltedTree/setup-rust/issues/157). This problem currently causes test failures on master (https://github.com/lf-lang/lingua-franca/runs/7951505913?check_suite_focus=true). It is caused by some internal bug of rustup and can be worked around by deleting its cache (Corrupt /missing manifests lead to installation conflicts rust-lang/rustup#2417). This change implements this such that if setting up rusts fails, we manually delete the cache and then try again.
2. Since Inclusion of reactor-rs as a submodule #1296 the rust runtime is included as a submodule. However, we forgot to update the CI setup for rust accordingly. This PR fixes the setup and correctly checks out the specified ref of the submodule