Legion python bindings not installed

[RAMitchell]

When legate.core is cloned and built for the first time the _skbuild directory does not yet exist and the following command returns None: https://github.com/nv-legate/legate.core/blob/6f0f2b1f9ba91be70fee3e40677b9860aa3180c8/install.py#L364

A consequence is the later call to install_legion_bindings does nothing: https://github.com/nv-legate/legate.core/blob/6f0f2b1f9ba91be70fee3e40677b9860aa3180c8/install.py#L170C14-L170C14

If install is run again a second time, the _skbuild directory will exist and the legion python bindings will be correctly installed.

This is blocking this PR for building docs, as a call to a legate library using the standard python interpreter fails: rapidsai/legate-boost#9