[skip-changelog] chore: fix codecov badge url

[Bikappa]

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• The PR has no duplicates (please search among the Pull Requests
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?

Set the Codecov badge url to point the master branch instead of main, so that the badge image display a valid coverage percentage.

What is the current behavior?

The coverage percentage is "unknown" in the badge

What is the new behavior?

Correct information is shown

Does this PR introduce a breaking change, and is titled accordingly?

No

Other information

[codecov]

Codecov Report

Base: 36.53% // Head: 36.49% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (e6d2638) compared to base (558130b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2022      +/-   ##
==========================================
- Coverage   36.53%   36.49%   -0.05%     
==========================================
  Files         228      228              
  Lines       19294    19294              
==========================================
- Hits         7050     7041       -9     
- Misses      11413    11420       +7     
- Partials      831      833       +2     

Flag	Coverage Δ
unit	36.49% <ø> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
arduino/monitor/monitor.go	41.05% <0.00%> (-6.32%)	⬇️
arduino/cores/packagemanager/package_manager.go	66.58% <0.00%> (+0.75%)	⬆️

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