Revert to helm release pipeline upload (#7045)

Since it's not possible to pass the entire build context (like the BUILDKITE_TAG, and it doesn't really make sense to pass the BUILDKITE_SOURCE) when triggering a pipeline, this commit reverts to uploading the helm release pipeline, which is entirely possible here, since both pipelines are in the same repository.

The decision to use the trigger step was made thinking it was the right way to reuse pipeline steps from another, ultimately it depends™.

.buildkite/pipeline-release.yml

@@ -29,10 +29,6 @@ steps:
         if: | # merge-main or tags
           ( build.branch == "main" && build.source != "schedule" )
           || build.tag != null
-        trigger: cloud-on-k8s-operator-helm-release
-        build:
-          message: "${BUILDKITE_MESSAGE}"
-          commit: "${BUILDKITE_COMMIT}"
-          branch: "${BUILDKITE_BRANCH}"
-          env:
-            HELM_DRY_RUN: false
+        command: buildkite-agent pipeline upload .buildkite/pipeline-release-helm.yml
+        env:
+          HELM_DRY_RUN: false