Reduce CI runs

[vyasr]

This PR cuts down on the the number testing jobs in PRs while attempting to retain similar coverage. It also enables fail-fast so that matrix jobs will not proceed to completion if any one of the tests fail.

Contributes to rapidsai/build-planning#5

[vyasr]

Will also apply fail-fast: true to the test jobs once we've finished discussions on the matrix.

[vyasr]

We discussed offline and the new matrix reflects the consensus @ajschmidt8 @bdice and I came to.

[vyasr]

The latest run on the cudf test PR verifies that this workflow is valid: https://github.com/rapidsai/cudf/actions/runs/8116036240?pr=15201

[vyasr]

New run based on setting v100/a100 appropriately: https://github.com/rapidsai/cudf/actions/runs/8116093123?pr=15201

[bdice]

Net changes to PRs:
conda C++ tests go from 4 jobs to 3.
conda Python tests go from 4 jobs to 3.
wheel tests go from 3 jobs to 2.

amd64 jobs go from 8 (3 conda C++, 3 conda Python, 2 wheel) to 5 (2 conda C++, 2 conda Python, 1 wheel)
arm64 jobs go from 3 to ... 3. 😬

Of course, we also have to account for custom jobs like docs and notebooks that also consume amd64 runners. That usually adds about 2 jobs to the amd64 runners, depending on the repository.

For this to improve CI times, we have to be waiting on amd64 (not arm64). That's currently true, from my observations, and I expect that reducing amd64 usage will be sufficient to improve CI queue times.

If you also consider that one of those amd64 jobs uses the "earliest" driver and thus comes from a separate pool of runners, this is an even more significant improvement. 👍

[ajschmidt8]

found two invalid combinations

[bdice]

#184 (comment)
https://github.com/rapidsai/cudf/actions/runs/8117888463/job/22191610519?pr=15201

Confirming the earlier hypothesis about imbalanced queues for latest/earliest drivers -- for this job's C++ tests, the amd64 "latest" job is still waiting on a runner after 45 minutes, but the amd64 "earliest" job started immediately.

[vyasr]

Aside from some unrelated failures, the test cudf job looks good now. @ajschmidt8 I think this is good to go when you're happy with it.