ci: fix test workflow

[tasshi-me]

https://github.com/tasshi-me/fitting-rs/actions/runs/8002351698/job/21855369381

error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/runner/.rustup/toolchains/nightly-x[8](https://github.com/tasshi-me/fitting-rs/actions/runs/8002351698/job/21855369381#step:12:9)6_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names -Zinstrument-coverage --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit status: 1)
  --- stderr
  error: unknown unstable option: `instrument-coverage`

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.40%. Comparing base (8079ccd) to head (77a6305).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
- Coverage   96.22%   95.40%   -0.82%     
==========================================
  Files           5        5              
  Lines         530      501      -29     
==========================================
- Hits          510      478      -32     
- Misses         20       23       +3     

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[tasshi-me]

https://github.com/mozilla/grcov?tab=readme-ov-file#example-how-to-generate-source-based-coverage-for-a-rust-project