Enable new Rust code coverage instrumentation

[zbraniecki]

grcov merged https://github.com/mozilla/grcov/issues/509#issuecomment-732817023 - it would be great to have access to it from the action.

[LiquidityC]

This is solvable manually for the time being: https://github.com/Blightmud/Blightmud/blob/dev/.github/workflows/coverage.yml

[kngwyu]

@LiquidityC Thank you for your nice example! We use it at https://github.com/PyO3/pyo3

[williamdes]

Thank you @LiquidityC ! I managed to make it work on Rust stable #141 (https://github.com/actions-rs/grcov/issues/141#issuecomment-1328294877)

[liufuyang]

Would be nice to keep this repo updated?