[CI] Make build-start-operator.sh fail fast on setup errors (#4893)

* [CI] Make build-start-operator.sh fail fast on setup errors

Closes #4879.

The script previously chained:

  IMG=kuberay/operator:nightly make docker-image &&
  kind load docker-image kuberay/operator:nightly &&
  echo "Deploying operator with test overrides ..."
  IMG=kuberay/operator:nightly make deploy-with-override

If `make docker-image` failed (e.g. transient 504 from registry-1.docker.io),
the `&&` chain short-circuited but `make deploy-with-override` still ran as
a separate command. The script returned the exit code of the last command
(0), so downstream `kubectl wait deployment kuberay-operator` then timed
out because the operator image was never built/loaded, hiding the real
cause.

Fix: enable `set -euo pipefail` and remove the `&&` chain so any failing
step aborts the script immediately and propagates a non-zero exit code to
callers. Also use the `:-0` default for IS_FROM_RAY_RELEASE_AUTOMATION so
`set -u` does not error when the variable is unset.

This affects all Buildkite e2e steps that source this script. Behavior is
unchanged on the success path; on failure, the real cause now surfaces
instead of a misleading readiness timeout downstream.

* [CI] Address bot feedback: drop set -u to avoid nounset leaking to callers

Both Codex and Cursor Bugbot flagged that `set -u` propagates from this
sourced script to the calling Buildkite shell. Downstream steps in
`.buildkite/test-e2e.yml` and `.buildkite/test-historyserver-e2e.yml`
reference `${KUBERAY_TEST_RAY_IMAGE}` without a default value, which
would error under nounset when the non-release path leaves the variable
unset.

Drop `-u` while keeping `-e` and `-o pipefail`, which still gives us the
fail-fast behavior needed to fix #4879 without breaking downstream
references. Add an inline comment explaining the constraint so this is
not reintroduced. Revert the unneeded `${IS_FROM_RAY_RELEASE_AUTOMATION:-0}`
default since nounset is no longer in effect.

* ci: retrigger build

The previous run failed in Test Apiserver E2E (nightly operator) on
TestCreateClusterAutoscaler, a known-flaky apiserver e2e test tracked
upstream (kuberay#3293, kuberay#3670). The failure is unrelated to this
PR's changes to build-start-operator.sh; the operator and apiserver both
started successfully in that run.

Retriggering CI via an empty commit to confirm the flake.

Author: EagleLo

.buildkite/build-start-operator.sh

@@ -1,20 +1,27 @@
 #!/bin/bash
+set -eo pipefail
 
 # This script is used to start the operator in the buildkite test-e2e steps.
 
 # When starting from the ray ci release automation, we want to install the latest
 # released version from helm as actual users might. Ray ci is also always expected
 # to kick off from the release branch so tests should match up accordingly.
+#
+# NOTE: Do not enable `set -u` (nounset) here. This script is sourced by the
+# Buildkite steps, so shell options propagate to the caller; downstream steps
+# reference $KUBERAY_TEST_RAY_IMAGE without a default and would fail under -u
+# when the variable is not set (e.g. in the non-release path).
 
 if [ "$IS_FROM_RAY_RELEASE_AUTOMATION" = 1 ]; then
     helm repo update
     echo "Installing helm chart with test override values (feature gates enabled as needed)"
     # NOTE: The override file is CI/test-only. It is NOT part of the released chart defaults.
     helm install kuberay-operator kuberay/kuberay-operator -f ../.buildkite/values-kuberay-operator-override.yaml
-    KUBERAY_TEST_RAY_IMAGE="rayproject/ray:nightly-extra-py310-cpu" && export KUBERAY_TEST_RAY_IMAGE
+    KUBERAY_TEST_RAY_IMAGE="rayproject/ray:nightly-extra-py310-cpu"
+    export KUBERAY_TEST_RAY_IMAGE
 else
-    IMG=kuberay/operator:nightly make docker-image &&
-    kind load docker-image kuberay/operator:nightly &&
+    IMG=kuberay/operator:nightly make docker-image
+    kind load docker-image kuberay/operator:nightly
     echo "Deploying operator with test overrides (feature gates via test-overrides overlay)"
     IMG=kuberay/operator:nightly make deploy-with-override
 fi