chore(ci): fix codecov main branch name

[jansegre]

Motivation

Codecov branch config has been wrong since we changed the default branch, this likely only affects coverage comparisons.

Acceptance Criteria

• Use master as codecov branch

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: 25 lines in your changes are missing coverage. Please review.

Comparison is base (68d1edc) 84.74% compared to head (9a37b94) 84.68%.
Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
- Coverage   84.74%   84.68%   -0.07%     
==========================================
  Files         259      264       +5     
  Lines       21939    22020      +81     
  Branches     2964     3367     +403     
==========================================
+ Hits        18593    18647      +54     
- Misses       2703     2721      +18     
- Partials      643      652       +9     

Files	Coverage Δ
hathor/builder/cli_builder.py	74.48% <100.00%> (+0.26%)	⬆️
hathor/builder/sysctl_builder.py	41.66% <ø> (ø)
hathor/consensus/consensus.py	97.89% <100.00%> (ø)
hathor/manager.py	81.58% <100.00%> (-0.10%)	⬇️
hathor/mining/block_template.py	94.33% <100.00%> (ø)
hathor/p2p/resources/status.py	100.00% <ø> (ø)
hathor/sysctl/runner.py	90.00% <ø> (ø)
hathor/transaction/base_transaction.py	94.16% <ø> (+0.18%)	⬆️
hathor/transaction/block.py	95.21% <ø> (-0.22%)	⬇️
hathor/transaction/token_creation_tx.py	99.28% <ø> (-0.02%)	⬇️
... and 12 more

... and 13 files with indirect coverage changes

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