ci: simplify jobs, fix coverage

[davidhewitt]

• merge macos and linux job definitions
• use more dtolnay/rust-toolchain instead of moonrepo/setup-rust for consistency
• use cargo-llvm-cov instead of coverage-prepare

[codspeed-hq]

CodSpeed Performance Report

Merging #161 will improve performances by 10.29%

_{Comparing dh/ci-cleanups (3497a46) with main (a004d5f)}

Summary

⚡ 1 improvements
✅ 72 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/ci-cleanups	Change
⚡	unicode_jiter_iter	8.4 µs	7.6 µs	+10.29%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.06%. Comparing base (a004d5f) to head (67887ec).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
- Coverage   91.90%   91.06%   -0.85%     
==========================================
  Files          12       12              
  Lines        2138     2138              
  Branches     2138     2138              
==========================================
- Hits         1965     1947      -18     
  Misses        109      109              
- Partials       64       82      +18     

see 5 files with indirect coverage changes

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