Fix CI checks by downloading plugin snapshots for testing from ci.opensearch.org

[cwperks]

Description

Fix CI checks by downloading plugin snapshots for testing from ci.opensearch.org

Similar to changes in opensearch-project/job-scheduler#877

Related Issues

Fixes CI failures seen in #1971

* What went wrong:
Could not determine the dependencies of task ':obsBwcCluster#fullRestartClusterTask'.
> Server returned HTTP response code: 402 for URL: https://aws.oss.sonatype.org/service/local/artifact/maven/redirect?r=snapshots&g=org.opensearch.plugin&a=opensearch-observability&v=2.20.0.0-SNAPSHOT&p=zip

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[mengweieric]

Thanks