fix: respect KUBECONFIG env var for external repo kubeconfig path

When consumers use the kind-bootstrap GitHub Action, the action sets
the KUBECONFIG env var to the kubeconfig file it creates. The Taskfile
previously ignored this env var and always fell back to the hardcoded
.test-infra/kubeconfig path, causing task commands to fail.

Now the external repo branch checks the KUBECONFIG env var first and
only falls back to .test-infra/kubeconfig if the env var is unset or
the file it points to does not exist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scotwells

Taskfile.yml

@@ -11,8 +11,10 @@ vars:
     sh: |
       if [ -f "cluster/kind-config.yaml" ] && [ -f "components/flux/kustomization.yaml" ]; then
         echo "./kubeconfig"  # In test-infra repo
+      elif [ -n "$KUBECONFIG" ] && [ -f "$KUBECONFIG" ]; then
+        echo "$KUBECONFIG"  # External repo: use KUBECONFIG env var if set and file exists
       else
-        echo ".test-infra/kubeconfig"  # External repo
+        echo ".test-infra/kubeconfig"  # External repo: default fallback
       fi
   # Detect if we're running in the test-infra repo or externally
   REPO_DIR: