ci: Split out the CI setups for mutation testing

[poljar]

This patch attempts to limit the times we run the full mutation tests. The full tests take 1h to complete so it's not feasible to run them as part of a pull-request CI job.

The original setup did attempt to limit this as well, but pushing additional commits to a pull request did trigger the full run as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.37%. Comparing base (3b1ac68) to head (ceb0ce2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #152 +/- ## ========================================== + Coverage 90.32% 90.37% +0.05% ========================================== Files 32 32 Lines 1788 1787 -1 ========================================== Hits 1615 1615 + Misses 173 172 -1 ```

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[Urgau]

Heads up, with the release of rust-lang/cargo#13913 (in nightly-2024-05-19). Cargo has now gain the ability to declare --check-cfg args directly inside the [lints] table with [lints.rust.unexpected_cfgs.check-cfg][^1]:

Cargo.toml:

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(fuzzing)'] }

Note that the diagnostic output of the lint has been updated to suggest the [lints] approach first. You can use it to guide you through the --check-cfg arguments that may need to be added.

[^1]: take effect on Rust 1.80 (current nightly), is ignored on Rust 1.79 (current beta), and produce an unused warning below

[poljar]

Heads up, with the release of rust-lang/cargo#13913 (in nightly-2024-05-19). Cargo has now gain the ability to declare --check-cfg args directly inside the [lints] table with [lints.rust.unexpected_cfgs.check-cfg]1:

Ah nice, thanks for letting us know.