[skip-changelog] Proper handling of stdout/stderr copy in subprocess

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?
  Should fix this problem: https://github.com/arduino/arduino-cli/runs/8158553770?check_suite_focus=true#step:6:1752

• What is the current behavior?
  sometimes the output of the subprocess doesn't reach the output buffer:

=== RUN   TestCompletionNoArgs
>>> Running: /home/runner/work/arduino-cli/arduino-cli/arduino-cli completion
<<< Run completed (err = exit status 1)
    completion_test.go:32: 
        	Error Trace:	/home/runner/work/arduino-cli/arduino-cli/internal/integrationtest/completion/completion_test.go:32
        	Error:      	"" does not contain "Error: accepts 1 arg(s), received 0"
        	Test:       	TestCompletionNoArgs
--- FAIL: TestCompletionNoArgs (0.01s)

• What is the new behavior?
  The issue above should be fixed

• Does this PR introduce a breaking change, and is titled accordingly?
  No

[codecov]

Codecov Report

Merging #1862 (b702e75) into master (67a62d9) will decrease coverage by 0.02%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master    #1862      +/-   ##
==========================================
- Coverage   36.65%   36.62%   -0.03%     
==========================================
  Files         231      231              
  Lines       19609    19610       +1     
==========================================
- Hits         7187     7182       -5     
- Misses      11596    11600       +4     
- Partials      826      828       +2     

Flag	Coverage Δ
unit	36.62% <45.45%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
executils/process.go	80.95% <ø> (ø)
internal/integrationtest/arduino-cli.go	83.19% <45.45%> (-2.47%)	⬇️

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