ci: update grcov to latest stable version

[pulsastrix]

The new stable release of grcov fixes the compilation issue related to the time crate, which we previously worked around by pulling in the branch from mozilla/grcov#1191.

Therefore, our previous workaround is no longer necessary, and we can use the stable version again.

[falko17]

PR itself is good to go, but for some reason the test_coverage CI is failing now.

[pulsastrix]

Apparently, the -Zprofile unstable flag has been removed, see rust-lang/rust#131829 and the feature's tracking issue rust-lang/rust#42524.

The (stable) replacement seems to be the instrument-coverage flag, I'll have a look at the required changes to get our pipeline working again using this flag.

[coveralls]

coverage: 86.555% (+1.3%) from 85.242% when pulling d2b3d706be1ac5370da493b610b8a08186386854 on ci_update_grcov into 3832486412e311bd2acd0f46fde0017068f4d215 on main.

[pulsastrix]

The pipeline is now switched over to using the newer (and stable) -C instrument-coverage flag, which enables source-based coverage reports.

For future reference, see:

• https://doc.rust-lang.org/nightly/rustc/instrument-coverage.html#instrumentation-based-code-coverage
• https://github.com/mozilla/grcov?tab=readme-ov-file#example-how-to-generate-source-based-coverage-for-a-rust-project