fix: don't overwrite lockfile when locking in pdm install

[frostming]

Fixes #2086

Signed-off-by: Frost Ming me@frostming.com

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.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (70f557d) 84.54% compared to head (4a319ce) 84.54%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2090   +/-   ##
=======================================
  Coverage   84.54%   84.54%           
=======================================
  Files         100      100           
  Lines        9648     9649    +1     
  Branches     2112     2112           
=======================================
+ Hits         8157     8158    +1     
  Misses       1034     1034           
  Partials      457      457           

Flag	Coverage Δ
unittests	84.31% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pdm/cli/commands/install.py	89.58% <100.00%> (+0.22%)	⬆️

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