Use scipy-openblas only when building the Python extension (#3033)

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.

Author: dweindl

cmake/AmiciFindBLAS.cmake

@@ -15,6 +15,11 @@ if(DEFINED ENV{AMICI_BLAS_USE_SCIPY_OPENBLAS})
       "Using AMICI_BLAS_USE_SCIPY_OPENBLAS=${AMICI_BLAS_USE_SCIPY_OPENBLAS} from environment variable."
   )
   set(AMICI_BLAS_USE_SCIPY_OPENBLAS $ENV{AMICI_BLAS_USE_SCIPY_OPENBLAS})
+elseif(NOT DEFINED AMICI_BLAS_USE_SCIPY_OPENBLAS
+       AND NOT AMICI_PYTHON_BUILD_EXT_ONLY)
+  # If were are not building the Python extension, it's unlikely that we want to
+  # use scipy-openblas
+  set(AMICI_BLAS_USE_SCIPY_OPENBLAS FALSE)
 endif()
 
 if((${BLAS} STREQUAL "MKL" OR DEFINED ENV{MKLROOT})