Allow to specify the config file through ARDUINO_CONFIG_FILE env

[alessio-perugini]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
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• 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)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Add ARDUINO_CONFIG_FILE env var, which as the same behaviour of the --config-file flag.

What is the current behavior?

Currently an user can only specify a specifc config file through the usage of --config-file flag.
It's not possible to specify the config file thorugh an env var.

What is the new behavior?

Adding the ARDUINO_CONFIG_FILE is now possible to specify either a specifc config file or a directory
containing the default configuration file.
If the cli is provied with both --config-file and the new env the flag takes precedence.

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

no

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0dbd871) 68.83% compared to head (863dd62) 68.91%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2488      +/-   ##
==========================================
+ Coverage   68.83%   68.91%   +0.07%     
==========================================
  Files         204      204              
  Lines       20465    20465              
==========================================
+ Hits        14088    14104      +16     
+ Misses       5222     5211      -11     
+ Partials     1155     1150       -5     

Flag	Coverage Δ
unit	68.91% <100.00%> (+0.07%)	⬆️

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