Macbuild: upx not needed

[m-reuter]

Minor changes:

• Mac build was passing --upx but upx is not installed leading to an error message
• It also should not use upx, so --upx is removed when calling install_fs_prunned.sh
• Added guard around upx block to first check if upx binary exists to avoid error
• Also bumped pyproject.toml version to next minor dev (2.6.0-dev)