Use stable for coverage measurement

[ctz]

ditto rustls/rustls#1486

[codecov]

Codecov Report

Merging #181 (9288245) into main (8d6a733) will increase coverage by 0.01%.
Report is 10 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   96.31%   96.33%   +0.01%     
==========================================
  Files          17       17              
  Lines        4510     4524      +14     
==========================================
+ Hits         4344     4358      +14     
  Misses        166      166              

Files Changed	Coverage Δ
src/end_entity.rs	100.00% <100.00%> (ø)
src/test_utils.rs	100.00% <100.00%> (ø)
src/verify_cert.rs	97.36% <100.00%> (-0.03%)	⬇️

... and 4 files with indirect coverage changes

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