reduce variant matrix size in CI

[bcressey]

Issue number:
N/A

Description of changes:
Adjust the list-variants action to pick the *-nvidia flavors of variants when they exist, since from a CI perspective they provide similar but greater coverage.

Remove the global upstream source fallback for nvidia variants since this should be covered by the package-level exception. This fixes a current issue where if a package is not present in the lookaside cache, the nvidia builds can succeed while all others fail.

Testing done:
Ran the equivalent script locally:

❯ ls -d */ | cut -d'/' -f 1 | grep -vE '^(shared|target)$' | sort | awk '$0 != x "-nvidia" && NR>1 {print x} {x=$0} END {print}' | jq -R -s 'split("\n")[:-1]
'
[
  "aws-dev",
  "aws-ecs-1-nvidia",
  "aws-ecs-2-nvidia",
  "aws-k8s-1.23-nvidia",
  "aws-k8s-1.24-nvidia",
  "aws-k8s-1.25-nvidia",
  "aws-k8s-1.26-nvidia",
  "aws-k8s-1.27-nvidia",
  "aws-k8s-1.28-nvidia",
  "aws-k8s-1.29-nvidia",
  "aws-k8s-1.30-nvidia",
  "metal-dev",
  "metal-k8s-1.25",
  "metal-k8s-1.26",
  "metal-k8s-1.27",
  "metal-k8s-1.28",
  "metal-k8s-1.29",
  "vmware-dev",
  "vmware-k8s-1.25",
  "vmware-k8s-1.26",
  "vmware-k8s-1.27",
  "vmware-k8s-1.28",
  "vmware-k8s-1.29",
  "vmware-k8s-1.30"
]

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.