Accept --cmake-executable also through an environment variable

[manopapad]

I've been trying to pass --cmake-executable to scikit-build in a robust way through pip install, but I've hit a wall. See nv-legate/legate.core#908 (comment) for more. It would be helpful if scikit-build could also accept this option through an environment variable. The discussion at pypa/pip#11859 (comment) outlines some of the caveats with trying to pass command line arguments through pip install.