[core] plugins: Correctly handle installed virtual envs on Linux

[cbentejac]

Description

This PR updates the initialization logic for the DirTreeProcessEnv class in meshroom/core/plugins.py to improve support for plugins that use virtual environments. The changes make the environment setup more robust by explicitly handling the presence of a venv directory and ensuring paths are correctly added for both standard and virtual environment layouts.

Prior to this PR, virtual environments were correctly handled on Windows (with specific venv layout for Windows) but were not taken into account for Linux, for which we were only relying on the standard "bin"/"lib"/"lib64" structure. Both layouts are now supported at the same time, meaning a plugin can theoretically have the following structure:

├── customPlugin
│   ├── meshroom
│   ├── bin
│   ├── lib
│   ├── venv
│   │   ├── bin
│   │   ├── lib

In practice, virtual environments are generally not used at the same time as the standard "bin"/"lib"/"lib64" layout.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.00%. Comparing base (0e46cbe) to head (d61ed6e).
⚠️ Report is 2 commits behind head on develop.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
meshroom/core/plugins.py	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2886      +/-   ##
===========================================
- Coverage    79.02%   79.00%   -0.03%     
===========================================
  Files           51       51              
  Lines         6856     6858       +2     
===========================================
  Hits          5418     5418              
- Misses        1438     1440       +2     

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

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.