Fix go2_example pixi build issue, fixes #15

[samontab-rapid]

Fixes #15

Resolves the Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy) error when building with pixi

Changes:

• Add numpy >= 1.26, < 2 dependency (this is the version go2_ros2_sdk uses)
• Add -DPython3_EXECUTABLE=$CONDA_PREFIX/bin/python3 to ensure CMake uses the correct Python
• Removed unitree_go package as go2_ros2_sdk doesn't define it and it was generating a warning

Result:
pixi run go2_example