Fix for zsh-specific env loading behavior

[derrickauyoung]

Original Issue

Fixed #2058

Root Cause

zsh runs .zshenv before .zshrc, so the export HOME=/real/home inside the generated .zshenv triggered before rez's .zshrc could be sourced, bypassing the context

Fix

ZDOTDIR is the correct zsh-native mechanism for redirecting dot file loading, and doesn't require touching HOME at all

• Updated shells.py UnixShell class to implement default HOME environment var setting for the HOME env var hijack
• Updated rezplugins/shells/zsh.py class to re-implement default methods to utilize ZDOTDIR instead

Thanks to @JeanChristopheMorinPerso and @ legomir for their initial diagnosis and background info on this problem.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: derrickauyoung / name: Derrick Auyoung (7dcb6ba)

[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.64%. Comparing base (d415b96) to head (8a3b027).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/shells.py	28.57%	5 Missing ⚠️
src/rezplugins/shell/zsh.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2111      +/-   ##
==========================================
- Coverage   60.65%   60.64%   -0.02%     
==========================================
  Files         164      164              
  Lines       20584    20592       +8     
  Branches     3579     3579              
==========================================
+ Hits        12485    12487       +2     
- Misses       7224     7229       +5     
- Partials      875      876       +1     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.