test: use mock environment variables in maa-dirs tests

[wangl-cc]

Change environment variables is not thread-safe, use a mock struct to avoid conflict. Because rust support ZST, so in normal case, there should be no overhead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.27%. Comparing base (7539f2f) to head (ae05cdc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #366      +/-   ##
==========================================
+ Coverage   66.24%   66.27%   +0.02%     
==========================================
  Files          42       42              
  Lines        3623     3626       +3     
==========================================
+ Hits         2400     2403       +3     
  Misses       1223     1223              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.