Fix glfw name when using system installed version

[JafarAbdi]

Fix finding the library when setting -DMJPC_USE_SYSTEM_GLFW=ON to use the system glfw, similar to https://github.com/google-deepmind/mujoco/blob/088079eff0450e32b98ee743141780ed68307506/simulate/cmake/SimulateDependencies.cmake#L77-L91

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