refactor: remove the keyword and syntax previously necessary to work with environment variables in favour of an additional optional property 'env'

[arthurpalves]

What does this PR do

• Remove the syntax necessary to work with environment variables in custom properties;
• Add optional property env to CustomProperty type, whose default value is set to false;
• Internally, decode optional Bool to non-optional isEnvironmentVariable.

How can it be tested

• Adhere to the latest changes in variants.yml
• Include a custom property whose value reads from environment variable:

 - name: sample_property
    value: NAME_OF_ENV_VAR
    env: true
    destination: project

• Switch to this respective variant variants switch --variant A_VARIANT
• IF iOS Variants.swift should contain struct Secrets with your encrypted static var.
• IF Android gradleScripts/variants.gradle should contain:

System.getenv('NAME_OF_ENV_VAR')

Task

resolves #121

Checklist:

• I ran make validation locally with success
• I have not introduced new bugs
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new errors

[codecov]

Codecov Report

Merging #124 (82caaee) into develop (c68e33f) will increase coverage by 0.32%.
The diff coverage is 81.81%.

@@             Coverage Diff             @@
##           develop     #124      +/-   ##
===========================================
+ Coverage    62.09%   62.42%   +0.32%     
===========================================
  Files           31       30       -1     
  Lines         1100     1099       -1     
===========================================
+ Hits           683      686       +3     
+ Misses         417      413       -4     

Impacted Files	Coverage Δ
...riantsCore/Factory/FastlaneParametersFactory.swift	28.37% <0.00%> (+1.10%)	⬆️
Sources/VariantsCore/Schemas/Configuration.swift	95.83% <95.45%> (-4.17%)	⬇️
...antsCore/Factory/Android/GradleScriptFactory.swift	89.33% <100.00%> (+0.87%)	⬆️
Sources/VariantsCore/Schemas/iOS/iOSVariant.swift	97.61% <100.00%> (ø)

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