Fix SWIG binding generation fails if cmake is ran from opensim-core

[adamkewley]

Encountered when building opensim-core from within its source dir. Error:

gmake[2]: *** No rule to make target 'Bindings/Python/Bindings/preliminaries.i', needed by 'Bindings/Python/simbody.py'.  Stop.
gmake[1]: *** [CMakeFiles/Makefile2:6844: Bindings/Python/CMakeFiles/_simbody.dir/all] Error 2

Invocation Example:

cmake -S . -LAH -B ../opensim-build -DOPENSIM_DEPENDENCIES_DIR=${PWD}/../deps-install -DOPENSIM_WITH_CASADI=OFF -DOPENSIM_WITH_TROPTER=OFF -DBUILD_JAVA_WRAPPING=ON -DBUILD_PYTHON_WRAPPING=ON -DSWIG_DIR=~/swig/share/swig -DSWIG_EXECUTABLE=~/swig/bin/swig -DCMAKE_BUILD_TYPE=Release -DOPENSIM_DOXYGEN_USE_MATHJAX=OFF

The failure is because execute_process's working directory is wherever cmake is ran, and this can cause extra entries to be added to the swig invocation

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• no need to update because minor

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