legacy: Fixed regression on compile --preprocess output

[cmaglie]

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What kind of change does this PR introduce?

Fixed a regression after legacy builder refactoring.

What is the current behavior?

Preprocessor output is always displayed.

What is the new behavior?

The preprocessor output is displayed only with --preprocess flag.

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

Other information

[alessio-perugini]

nice catch!

[codecov]

Codecov Report

Patch coverage: 53.84% and project coverage change: -0.01% ⚠️

Comparison is base (5a4f48b) 63.06% compared to head (6f52439) 63.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2339      +/-   ##
==========================================
- Coverage   63.06%   63.06%   -0.01%     
==========================================
  Files         201      201              
  Lines       19259    19259              
==========================================
- Hits        12146    12145       -1     
- Misses       6059     6060       +1     
  Partials     1054     1054              

Flag	Coverage Δ
unit	63.06% <53.84%> (-0.01%)	⬇️

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

Files Changed	Coverage Δ
commands/compile/compile.go	68.85% <50.00%> (-0.14%)	⬇️
arduino/builder/builder.go	71.42% <57.14%> (-0.18%)	⬇️

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