sim.pysim: Only close VCD/GTKW files if we opened them ourselves.

[wanda-phi]

Fixes #1107.

[codecov]

Codecov Report

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

Project coverage is 85.90%. Comparing base (a586df8) to head (04be176).

Files	Patch %	Lines
amaranth/sim/pysim.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1156   +/-   ##
=======================================
  Coverage   85.89%   85.90%           
=======================================
  Files          43       43           
  Lines        8687     8691    +4     
  Branches     2067     2067           
=======================================
+ Hits         7462     7466    +4     
  Misses       1024     1024           
  Partials      201      201           

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[whitequark]

Thanks!