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.