…#641)

This PR includes the following changes:
- Adds a `libraft-tests` package to the `libraft-split` recipe
  - This is a prerequisite for removing "Project Flash" from our build/CI scripts
   - The `libraft-tests` package was added as an additional output to the `libraft-split` recipe
- Consolidates remaining C++ recipes into `libraft-split` recipe
   - This gets rid of a lot of duplicate code between the recipes and reduces the number of times we have to call conda build in our CI scripts
- Migrate the "from sources" builds done in GPU tests by building packages using conda. This is done for the following reasons:
  - This is required step to improve the Ops CI/CD setup to a more convenient pipeline
  - This is required to start using conda compilers and `mamba` to build RAPIDS packages
  - This prevent us from manually managing and installing the dependencies in GPU job
  - This ensure the packages can be installed (no conda conflict while installing)
  - This ensure the tests are running and working against the package content and not the build results. Currently the Python packages are not tested

Dependency version specs are now specified in `conda/recipes/libraft/conda_build_config.yaml`. The exception here is the version spec used for cudatoolkit since that comes from an environment variable in the CI process.

Authors:
  - Jordan Jacobelli (https://github.com/Ethyling)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: rapidsai#641

`CMAKE_ARGS` is a special environment variable in conda-forge's build process, so we need to avoid shadowing it.

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: rapidsai#701