graphIO: serialize username that saved the file

[Alxiice]

Description

In the serialized file, save the username that last saved the file.
It could be interesting for stat purposes, we can get the modification date with statbut not the username easily.

[gemini-code-assist]

Code Review

This pull request adds the current username to the serialized file header using getpass.getuser(). Feedback suggests wrapping the getpass.getuser() call in a try-except block to prevent potential crashes in environments where user information is unavailable, such as containerized or headless systems.

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.06%. Comparing base (1d89d80) to head (6838825).
⚠️ Report is 14 commits behind head on develop.

Files with missing lines	Patch %	Lines
meshroom/core/graphIO.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3134      +/-   ##
===========================================
- Coverage    86.07%   86.06%   -0.01%     
===========================================
  Files           78       78              
  Lines        12111    12117       +6     
===========================================
+ Hits         10425    10429       +4     
- Misses        1686     1688       +2     

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