Fix board search not returning boards if installed core is not latest version

[silvanocerza]

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

Fixes a bug with an existing command and related gRPC function.

• What is the current behavior?

board search command and related gRPC function don't return board FQBNs belonging to an installed core if it's not the latest version available.

$ arduino-cli core list
ID           Installed Latest Name                                        
arduino:samd 1.8.10    1.8.11 Arduino SAMD Boards (32-bits ARM Cortex-M0+)

$ arduino-cli board search arduino:samd:mkrwifi1010
Board Name FQBN Platform ID 

$ arduino-cli board search mkr wifi 1010
Board Name            FQBN Platform ID  
Arduino MKR WiFi 1010      arduino:samd

• What is the new behavior?

board search command and related gRPC function now correctly return all FQBNs of installed cores even if it's not the latest version available.

$ arduino-cli core list
ID           Installed Latest Name                                        
arduino:samd 1.8.10    1.8.11 Arduino SAMD Boards (32-bits ARM Cortex-M0+)

$ arduino-cli board search arduino:samd:mkrwifi1010
Board Name            FQBN                     Platform ID  
Arduino MKR WiFi 1010 arduino:samd:mkrwifi1010 arduino:samd 

$ arduino-cli board search mkr wifi 1010           
Board Name            FQBN                     Platform ID  
Arduino MKR WiFi 1010 arduino:samd:mkrwifi1010 arduino:samd 

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

Nope.

• Other information:

Partly fixes #1401.

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