Fix crash in linux.environ plugin when environ file is missing

[B0TAxy]

This MR fixes a bug in the linux.environ plugin where attempting to parse the environment of a process would crash if the /proc/<pid>/environ file does not exist (e.g., kernel threads or race conditions during process termination).

Changes:

• dissect/target/plugins/os/unix/linux/proc.py: Added an explicit .exists() check in _parse_environ before attempting to read the file. It now gracefully returns/skips instead of raising an exception.
• tests/conftest.py: Updated the fs_linux_proc fixture to simulate a scenario where a process in /proc lacks an environ file (conditional mapping).
• tests/.../test_environ.py:
• Updated the expected result count from 4 to 3 (since one process is now correctly skipped due to the missing file).
• Added a linter suppression comment for the fixture usage.

Testing:

• Updated existing tests to verify that the plugin no longer crashes on missing files and correctly parses the remaining valid processes.

[B0TAxy]

Hi @Schamper & @Horofic , checking in on this as it's been open for a few weeks. Since this is a minor fix, I was hoping we could get it in soon. Is there anything blocking this (or anything else needed from my side)?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.93%. Comparing base (4ec38bf) to head (53e59ee).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1504      +/-   ##
==========================================
- Coverage   80.94%   80.93%   -0.02%     
==========================================
  Files         397      397              
  Lines       34892    34893       +1     
==========================================
- Hits        28244    28240       -4     
- Misses       6648     6653       +5     

Flag	Coverage Δ
unittests	80.93% <100.00%> (-0.02%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 11 untouched benchmarks

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_{Comparing B0TAxy:fix/environ_file_not_found_error (53e59ee) with main (4ec38bf)}