clarify which dependencies in dependencies.yaml are conda-only

[jameslamb]

This project has some dependencies in dependencies.yaml which are part of groups that have output_types: requirements, but which use version specifiers that aren't recognized by pip.

For example, the use of a secondary build component matching a build string (a conda-specific pattern):

https://github.com/rapidsai/wholegraph/blob/f85ee4356f2e3d42195a2e0a6c7f195154c47091/dependencies.yaml#L247

And the use of a single = pin (not recognized by pip)

https://github.com/rapidsai/wholegraph/blob/f85ee4356f2e3d42195a2e0a6c7f195154c47091/dependencies.yaml#L288

I believe these were intended to only affect conda outputs from rapids-dependency-file-generator. This marks them that way.

Notes for Reviewers

I discovered this while running the following cuda11.8-pip unified devcontainer from https://github.com/rapidsai/devcontainers.

rapids-make-pip-env --force

That resulted in an error like this when wholegraph was included.

ERROR: Invalid requirement: 'pytorch-cuda=11.8': Expected end or semicolon (after name and no valid version specifier)
    pytorch-cuda=11.8
                ^ (from line 75 of /tmp/rapids.requirements.txt)
Hint: = is not a valid operator. Did you mean == ?

[jameslamb]

/merge