Calculate miri test coverage of unsafe code

[arrayka]

The idea is this:
Clone Generate code coverage step in .github/workflows/ci.yml under code coverage workflow.
Name the clone Generate miri code coverage.
In the clone, add RUSTFLAGS="--cfg miri" before cargo llvm-cov command to calculate miri test coverage.
Save it as lcov_miri.info

Upload two files to codecov: lcov.info, lcov_miri.info

Expectation is that codecov will output coverages for both test modes: regular unit tests and miri.
The same coverage thresholds apply to miri test coverage.