Fix cach file path's type to Path | str

[drunkwcodes]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

I'm writing tests for caches.py, and I find that using str directory path is more convenient.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (7582b92) 84.27% compared to head (8d3ad9f) 84.27%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2192   +/-   ##
=======================================
  Coverage   84.27%   84.27%           
=======================================
  Files         100      100           
  Lines        9787     9787           
  Branches     2161     2161           
=======================================
  Hits         8248     8248           
  Misses       1065     1065           
  Partials      474      474           

Flag	Coverage Δ
unittests	84.05% <100.00%> (ø)

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Files Changed	Coverage Δ
src/pdm/models/caches.py	88.42% <100.00%> (ø)

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