Fix Codecov uploading from GitHub Actions

[mattrubin]

The post-v1 Codecov action doesn't automatically find coverage files from Xcode tests.

[codecov]

Codecov Report

Merging #246 (2c9e8f4) into develop (57d2ba9) will increase coverage by 3.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #246      +/-   ##
===========================================
+ Coverage    94.06%   97.16%   +3.09%     
===========================================
  Files            6        6              
  Lines          354      388      +34     
===========================================
+ Hits           333      377      +44     
+ Misses          21       11      -10     

Impacted Files	Coverage Δ
Sources/Generator.swift	100.00% <0.00%> (ø)
Sources/Token+URL.swift	98.41% <0.00%> (+0.15%)	⬆️
Sources/Keychain.swift	92.56% <0.00%> (+0.67%)	⬆️
Sources/Crypto.swift	100.00% <0.00%> (+32.25%)	⬆️

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