re-enable mixing libcuspatial wheels with libcudf conda packages

[jameslamb]

Description

Fixes #1455

devcontainer conda CI jobs are failing in this project because of the following mix of characteristics for thoes jobs:

• all build and runtime dependencies for libcuspatial, cuspatial, cuproj are installed via conda
• libcuspatial, cuspatial, and cuproj wheels are then built with pip install -e --no-deps --no-build-isolation
• import libcuspatial results in unconditionally running import libcudf
• libcudf is provided by the libcudf conda package, which does not have any Python modules, so that import fails

This fixes that, and restores the ability to mix a pip install'd cuspatial / cuproj with a conda-installed libcudf.

Notes for Reviewers

How did CI not catch this before?

When rapidsai/devcontainers#387 was merged, I only re-ran the pip devcontainers CI job on #1450.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

/merge

[bdice]

Unblocks CI for #1453, so I merged this.