integration test: Added function to run arduino-cli with custom env

[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?
  Added an helper function to run arduino-cli with custom env

• What is the current behavior?
  No changes

• What is the new behavior?
  No changes

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

[codecov]

Codecov Report

Base: 36.66% // Head: 36.66% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (de86966) compared to base (736608f).
Patch coverage: 71.42% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1877      +/-   ##
==========================================
- Coverage   36.66%   36.66%   -0.01%     
==========================================
  Files         231      231              
  Lines       19666    19680      +14     
==========================================
+ Hits         7211     7216       +5     
- Misses      11627    11635       +8     
- Partials      828      829       +1     

Flag	Coverage Δ
unit	36.66% <71.42%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
internal/integrationtest/arduino-cli.go	82.57% <71.42%> (-1.43%)	⬇️
arduino/cores/packagemanager/package_manager.go	65.82% <0.00%> (-0.76%)	⬇️

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[MatteoPologruto]

Thanks Cristian!