Use scipy-openblas only when building the Python extension

[dweindl]

Fixes a flaw in cmake/AmiciFindBLAS.cmake that resulted in incorrectly trying to use scipy-openblas during standalone C++ builds. Now, unless explicitly requested, scipy-openblas will only be considered when building (only) the Python extension.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.40%. Comparing base (b0b2684) to head (a699c60).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3033      +/-   ##
==========================================
- Coverage   78.06%   77.40%   -0.67%     
==========================================
  Files         311      311              
  Lines       20587    20587              
  Branches     1499     1499              
==========================================
- Hits        16071    15935     -136     
- Misses       4506     4642     +136     
  Partials       10       10              

Flag	Coverage Δ
cpp	70.33% <ø> (-0.03%)	⬇️
cpp_python	36.75% <ø> (ø)
petab	45.72% <ø> (ø)
petab_sciml	13.66% <ø> (ø)
python	69.33% <ø> (-0.03%)	⬇️
sbmlsuite-jax	?

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see 7 files with indirect coverage changes

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