turn off package proxy cache on conda-based jobs

[jameslamb]

This was originally opened for testing: https://github.com/rapidsai/rapids-multi-gpu/pull/72#issuecomment-2660149987

But then expanded to turn off the proxy cache for all conda-based jobs that run on self-hosted runners, for issues like this:

rapidsai/kvikio#603 (comment)

[jameslamb]

This branch shouldn't be deleted until this is merged: https://github.com/rapidsai/rapids-multi-gpu/pull/76

[bdice]

We might need to turn off the cache for conda C++ test jobs too. We are seeing problems in cuDF C++ tests, which can't find the latest nvcomp packages.

https://github.com/nv-gha-runners/roadmap/issues/192
https://github.com/rapidsai/cudf/actions/runs/13460614745/job/37616774217?pr=17944#step:9:79

[jameslamb]

We might need to turn off the cache for conda C++ test jobs too

Sure. This PR was only opened for testing https://github.com/rapidsai/rapids-multi-gpu/pull/72, which is now closed. And rapids-multi-gpu is no longer relying on it, as of https://github.com/rapidsai/rapids-multi-gpu/pull/76.

If we want to re-purpose this branch to actually change where the proxy cache is used in RAPIDS, that's fine.