Increase priority of manually-installed platform.

[cmaglie]

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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)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Makes the manually-installed platform always preferred over the platforms installed through Board Manager

What is the current behavior?

TODO

What is the new behavior?

TODO

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

Other information

Draft PR to run the test-suite

[codecov]

Codecov Report

Attention: Patch coverage is 89.11565% with 16 lines in your changes missing coverage. Please review.

Project coverage is 67.80%. Comparing base (5f84ecb) to head (dad5063).
Report is 50 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/arduino/cores/packagemanager/loader.go	86.07%	5 Missing and 6 partials ⚠️
internal/cli/core/list.go	50.00%	2 Missing and 1 partial ⚠️
.../arduino/cores/packagemanager/install_uninstall.go	50.00%	0 Missing and 1 partial ⚠️
...al/arduino/cores/packagemanager/package_manager.go	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2810      +/-   ##
==========================================
+ Coverage   67.74%   67.80%   +0.06%     
==========================================
  Files         238      238              
  Lines       22410    22416       +6     
==========================================
+ Hits        15181    15200      +19     
+ Misses       6034     6024      -10     
+ Partials     1195     1192       -3     

Flag	Coverage Δ
unit	67.80% <89.11%> (+0.06%)	⬆️

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