Fix missing time unit suffix on wait_timeout default

The kind-bootstrap action's wait_timeout input defaulted to "300"
(bare number) which newer kubectl rejects as an invalid duration.
Changed to "300s" to match the Taskfile's own default.

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

Author: scotwells

.github/actions/kind-bootstrap/action.yaml

@@ -8,9 +8,9 @@ inputs:
     required: false
     default: test-infra
   wait_timeout:
-    description: "Seconds to wait for add-ons to become Ready"
+    description: "Time to wait for add-ons to become Ready (e.g. 300s, 5m)"
     required: false
-    default: "300"
+    default: "300s"
   images:
     description: "Space-separated list of Docker images to load into kind (optional)"
     required: false