Tweak mise install and lockfile handling in copier-python update action

[smkent]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.00%. Comparing base (e99455f) to head (8320375).

Files with missing lines	Patch %	Lines
copier_python/update.py	40.00%	3 Missing ⚠️
copier_python/repo.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   85.64%   84.00%   -1.65%     
==========================================
  Files           4        4              
  Lines         216      225       +9     
  Branches       19       20       +1     
==========================================
+ Hits          185      189       +4     
- Misses         21       25       +4     
- Partials       10       11       +1     

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