Refactor HostSystem to improve maintainability

[ascopes]

• Scope per Mojo execution rather than per session for consistency with the rest of the project.
• Use functional programming style for project property and env parsing.
• Rescope test constructor.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.12%. Comparing base (e333aeb) to head (34dfbd9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
+ Coverage   90.99%   91.12%   +0.14%     
==========================================
  Files          48       48              
  Lines        1497     1520      +23     
  Branches      100      100              
==========================================
+ Hits         1362     1385      +23     
  Misses         99       99              
  Partials       36       36              

Files with missing lines	Coverage Δ
.../ascopes/protobufmavenplugin/utils/HostSystem.java	100.00% <100.00%> (ø)

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