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

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%)	⬆️

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

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

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