Conditionally modify envvar vs. global CMAKE_PREFIX_PATH in rapids_cmake_support_conda_env

[trxcllnt]

Description

This PR updates the logic when rapids_cmake_support_conda_env() is called with MODIFY_PREFIX_PATH.

The current logic prepends the $PREFIX/$BUILD_PREFIX/$CONDA_PREFIX paths to the global CMAKE_PREFIX_PATH variable.

This PR proposes the following changes:

1. Append instead of prepend the $PREFIX/$BUILD_PREFIX/$CONDA_PREFIX paths
2. Prefer modifying the CMAKE_PREFIX_PATH environment variable instead of the global CMAKE_PREFIX_PATH variable
3. Only modify the global CMAKE_PREFIX_PATH variable if it's already populated (preserving paths in ENV{CMAKE_PREFIX_PATH})

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[robertmaynard]

We should prefer usingDEFINED since it makes it clearer.

If some user some how defines CMAKE_PREFIX_PATH to a empty string, they win an award :)

[trxcllnt]

/merge