Use stable for coverage measurement

[ctz]

The limitation on this is that nightly was required to run doctests, however I don't believe those are contributing much incremental coverage.

[codecov]

Codecov Report

Merging #1486 (d78f978) into main (cb9884d) will increase coverage by 0.06%.
Report is 18 commits behind head on main.
The diff coverage is 98.00%.

@@            Coverage Diff             @@
##             main    #1486      +/-   ##
==========================================
+ Coverage   96.43%   96.49%   +0.06%     
==========================================
  Files          72       72              
  Lines       15161    15114      -47     
==========================================
- Hits        14620    14585      -35     
+ Misses        541      529      -12     

Files Changed	Coverage Δ
rustls/src/msgs/enums.rs	100.00% <ø> (ø)
rustls/src/webpki/verify.rs	99.44% <ø> (ø)
rustls/src/client/builder.rs	87.83% <88.46%> (-0.40%)	⬇️
rustls/src/client/client_conn.rs	86.41% <94.11%> (+0.42%)	⬆️
rustls/src/crypto/ring/mod.rs	95.83% <97.14%> (+1.58%)	⬆️
rustls/src/builder.rs	99.00% <100.00%> (+0.05%)	⬆️
rustls/src/client/hs.rs	97.21% <100.00%> (+0.15%)	⬆️
rustls/src/client/tls12.rs	98.53% <100.00%> (+0.14%)	⬆️
rustls/src/client/tls13.rs	97.05% <100.00%> (-0.01%)	⬇️
rustls/src/crypto/mod.rs	100.00% <100.00%> (+100.00%)	⬆️
... and 11 more

... and 8 files with indirect coverage changes

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