chore: prevent empty codecov report

[Bikappa]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• 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?

The CI pipeline is changed in order to prevent empty Codecov coverage reports. It follows the workaround in the Codecov related issue

What is the current behavior?

Commit related Codecov reports are empty, see example screenshot:

What is the new behavior?

Coverage reports are shown correctly by Codecov

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

Other information

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Base: 0.00% // Head: 36.47% // Increases project coverage by +36.47% 🎉

Coverage data is based on head (f8bdc39) compared to base (357d465).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2020       +/-   ##
===========================================
+ Coverage        0   36.47%   +36.47%     
===========================================
  Files           0      228      +228     
  Lines           0    19294    +19294     
===========================================
+ Hits            0     7038     +7038     
- Misses          0    11422    +11422     
- Partials        0      834      +834     

Flag	Coverage Δ
unit	36.47% <ø> (?)

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

Impacted Files	Coverage Δ
...egacy/builder/print_used_and_not_used_libraries.go	100.00% <0.00%> (ø)
cli/core/upgrade.go	0.00% <0.00%> (ø)
commands/board/search.go	0.00% <0.00%> (ø)
arduino/resources/download.go	62.50% <0.00%> (ø)
cli/lib/update_index.go	0.00% <0.00%> (ø)
arduino/builder/compilation_database.go	58.97% <0.00%> (ø)
arduino/libraries/librariesindex/reference.go	100.00% <0.00%> (ø)
version/version.go	64.28% <0.00%> (ø)
i18n/i18n.go	40.00% <0.00%> (ø)
cli/lib/download.go	0.00% <0.00%> (ø)
... and 218 more

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