[breaking] Fix regression in core caching

[cmaglie]

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What kind of change does this PR introduce?

Fix a regression in compiled core caching. A wrong precompiled core may be selected if build options are changed.

What is the current behavior?

The same "cached" core is wrongly used:

$ arduino-cli compile -b arduino:avr:mega -v | grep -i precompiled
Using precompiled core: /tmp/arduino/cores/arduino_avr_mega_f702e3425f7b051e4211596c91e0b47a/core.a
$ arduino-cli compile -b arduino:avr:mega:cpu=atmega1280 -v | grep -i precompiled
Using precompiled core: /tmp/arduino/cores/arduino_avr_mega_f702e3425f7b051e4211596c91e0b47a/core.a

What is the new behavior?

$ arduino-cli compile -b arduino:avr:mega -v | grep -i precompiled
Using precompiled core: /tmp/arduino/cores/arduino_avr_mega_f702e3425f7b051e4211596c91e0b47a/core.a
$ arduino-cli compile -b arduino:avr:mega:cpu=atmega1280 -v | grep -i precompiled
Using precompiled core: /tmp/arduino/cores/arduino_avr_mega_cpu_atmega1280_f702e3425f7b051e4211596c91e0b47a/core.a

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

There is a change in the golang API, no visible change for users.

Other information

Fix: https://github.com/arduino/arduino-ide/issues/1990

See also:
https://forum.arduino.cc/t/teensy-compile-sketch-fails-clear-out-temp-build-completes/1110104/6
https://forum.pjrc.com/threads/72572-Teensyduino-1-59-Beta-2?p=324071&viewfull=1#post324071

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (ea066cc) 62.57% compared to head (474ffac) 62.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2145      +/-   ##
==========================================
+ Coverage   62.57%   62.59%   +0.02%     
==========================================
  Files         227      227              
  Lines       19538    19540       +2     
==========================================
+ Hits        12225    12231       +6     
+ Misses       6216     6213       -3     
+ Partials     1097     1096       -1     

Flag	Coverage Δ
unit	62.59% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
arduino/cores/board.go	93.65% <100.00%> (ø)
arduino/cores/packagemanager/package_manager.go	74.71% <100.00%> (ø)
legacy/builder/phases/core_builder.go	74.28% <100.00%> (+4.38%)	⬆️

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