[devel-CMake] Fix Python interpreter not defined.

[shishaochen]

With CMake built-in API FindPython, user can avoid explicity decaring variable PYTHON_EXECUTABLE.

[codecov-commenter]

Codecov Report

Merging #776 (381b605) into devel (f326a86) will decrease coverage by 7.63%.
The diff coverage is n/a.

❗ Current head 381b605 differs from pull request most recent head 3877480. Consider uploading reports for the commit 3877480 to get more accurate results

@@            Coverage Diff             @@
##            devel     #776      +/-   ##
==========================================
- Coverage   74.30%   66.66%   -7.64%     
==========================================
  Files          82        6      -76     
  Lines        6534       15    -6519     
==========================================
- Hits         4855       10    -4845     
+ Misses       1679        5    -1674     

Impacted Files	Coverage Δ
deepmd/infer/deep_wfc.py
deepmd/descriptor/loc_frame.py
deepmd/__init__.py
deepmd/entrypoints/__init__.py
deepmd/entrypoints/freeze.py
deepmd/utils/argcheck.py
deepmd/loggers/__init__.py
deepmd/train/run_options.py
deepmd/utils/data.py
source/op/_soft_min_virial_grad.py
... and 64 more

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