Set installed property for installed platform when core search is executed

[MatteoPologruto]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Code enhancement

What is the current behavior?

The installed property is never set.

./arduino-cli core install arduino:avr --format json 

./arduino-cli core search "" --format json | jq '.[] | select(.id == "arduino:avr") | .installed' null 

What is the new behavior?

The installed property is set if the platform is installed.

./arduino-cli core install arduino:avr --format json 

./arduino-cli core search "" --format json | jq '.[] | select(.id == "arduino:avr") | .installed' "1.8.6" 

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

No

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (9bc2afc) 62.95% compared to head (641f64e) 62.95%.

Additional details and impacted files

@@ Coverage Diff @@## master #2223 +/- ## ======================================= Coverage 62.95% 62.95% ======================================= Files 220 220 Lines 19476 19478 +2 ======================================= + Hits 12261 12263 +2  Misses 6132 6132 Partials 1083 1083 

Flag	Coverage Δ
unit	62.95% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
commands/core/search.go	84.21% <100.00%> (+0.57%)	⬆️

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