Fallback mount sysvol to C: if not mounted to another drive letter

[Schamper]

Fixes #666

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 74.90%. Comparing base (52c4480) to head (e2d076c).

Files	Patch %	Lines
dissect/target/plugins/os/windows/_os.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #671   +/-   ##
=======================================
  Coverage   74.90%   74.90%           
=======================================
  Files         288      288           
  Lines       24109    24115    +6     
=======================================
+ Hits        18059    18064    +5     
- Misses       6050     6051    +1     

Flag	Coverage Δ
unittests	74.90% <83.33%> (+<0.01%)	⬆️

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[Poeloe]

LGTM!