fix: Sketch preprocessing errors were displayed on stdout instead of stderr

[cmaglie]

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What kind of change does this PR introduce?

Fix the error output during sketch preprocessing wrongly redirected to stdout instead of stderr.

What is the current behavior?

$ arduino-cli compile -b arduino:avr:uno 
/home/megabug/Arduino/Blink/Blink.ino:1:2: error: #error eeee
 #error eeee
  ^~~~~
$ arduino-cli compile -b arduino:avr:uno 1> /dev/null 
Error during build: exit status 1

What is the new behavior?

$ arduino-cli compile -b arduino:avr:uno 
/home/megabug/Arduino/Blink/Blink.ino:1:2: error: #error eeee
 #error eeee
  ^~~~~
$ arduino-cli compile -b arduino:avr:uno 1> /dev/null 
/home/megabug/Arduino/Blink/Blink.ino:1:2: error: #error eeee
 #error eeee
  ^~~~~
Error during build: exit status 1

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

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.61%. Comparing base (d1b47cf) to head (5810d62).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2806      +/-   ##
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- Coverage   67.63%   67.61%   -0.02%     
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  Files         238      238              
  Lines       22392    22392              
==========================================
- Hits        15144    15141       -3     
- Misses       6060     6062       +2     
- Partials     1188     1189       +1     

Flag	Coverage Δ
unit	67.61% <100.00%> (-0.02%)	⬇️

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[alessio-perugini]

LGTM, tested also locally on my machine