testing: import virtualenv lazily

[efiop]

Fixes #10118

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (322fcec) 90.55% compared to head (a1fa3ce) 90.30%.

Files	Patch %	Lines
dvc/testing/benchmarks/fixtures.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10120      +/-   ##
==========================================
- Coverage   90.55%   90.30%   -0.25%     
==========================================
  Files         488      488              
  Lines       37395    37395              
  Branches     5435     5435              
==========================================
- Hits        33864    33771      -93     
- Misses       2895     2964      +69     
- Partials      636      660      +24     

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