Code coverage for the Rust code

[imobachgs]

Problem

We do not have any report about the test coverage for the Rust code.

Solution

This PR enables code coverage reporting for the Rust code using Tarpaulin.

• All jobs are using coverallsapp/github-action@v2 now.
• Use cargo-binstall to install Tarpaulin, saving the build time. An alternative approach might be to package Tarpaulin ourselves.
• rust_ci uses the yast-ruby container because the Coveralls GitHub Action was not working with the Tumbleweed image (IDK the reason).
• rust_ci uses the cobertura format instead of lcov because the file names in the latter are absolute (which looks wrong). See the list of supported formats if you are interested.
• Code coverage is ~15% 😞

Screenshots

[coveralls]

coverage: 72.595% (-4.3%) from 76.846% when pulling 9336b1a on rust-coverage into 0fde6ba on master.

[jreidinger]

so we have third way to install rust stuff? rustup component, cargo install and now cargo-binstall? It start to be quite messy

[imobachgs]

Just to clarify: rustup is for managing the rust toolchain. cargo install is for binary crates (building for sources) and cargo-binstall to avoid building for sources.