Only run backport workflow it a backport label was added.

[dblock]

Signed-off-by: dblock dblock@dblock.org

Description

In #5143 the backport workflow ran because I added a skip-changelog label.

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[dblock]

@VachaShah I think we might need this in a bunch of projects as we also are rolling out changelog labelling.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/6550/
• CommitID: e923703
  Please examine the workflow log, locate, and copy-paste the failure below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[dblock]

Doesn't seem to work :( Found https://stackoverflow.com/questions/71523153/github-actions-on-pull-request-merged-with-speciphic-labels for the same.

This skips non-backport labels correctly, so it's some progress.

if: ${{ contains(github.event.label.name, 'backport ') 

But this still runs on non-merged PRs, not sure why.

if: ${{ contains(github.event.label.name, 'backport ') && (github.event.pull_request.merged == true) }}

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/6551/
• CommitID: 2b0743c

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/6552/
• CommitID: c51f0c3

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/6554/
• CommitID: 2494cc0
  Please examine the workflow log, locate, and copy-paste the failure below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[dblock]

Tests with failures:
 - org.opensearch.clustermanager.ClusterManagerTaskThrottlingIT.testTimeoutWhileThrottling
 - org.opensearch.persistent.PersistentTasksExecutorFullRestartIT.testFullClusterRestart

#5083
#5145