benchmark: fix benchmark for reflink

[skshetry]

Now we create tmpdir inside .pytest_cache directory and run the test there, same as we do in dvc-objects.
We probably should write similar tests in dvc, as most of our tests run in tmpdir which is usually tmpfs in linux.

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (45c8588) 63.79% compared to head (910e9b8) 63.73%.

Files	Patch %	Lines
tests/benchmarks/test_checkout.py	37.50%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #469      +/-   ##
==========================================
- Coverage   63.79%   63.73%   -0.06%     
==========================================
  Files          61       61              
  Lines        4118     4128      +10     
  Branches      706      708       +2     
==========================================
+ Hits         2627     2631       +4     
- Misses       1345     1351       +6     
  Partials      146      146              

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