Add source common.sh in release-manager.sh

[mrodm]

What is the problem this PR solves?

// Please do not just reference an issue. Explain WHAT the problem this PR solves here.

+ retry 3 docker pull --quiet docker.elastic.co/infra/release-manager:latest
./.buildkite/scripts/release-manager.sh: line 30: retry: command not found
+ echo 'Error pulling docker.elastic.co/infra/release-manager:latest Docker image, we continue'
Error pulling docker.elastic.co/infra/release-manager:latest Docker image, we continue

How does this PR solve the problem?

// Explain HOW you solved the problem in your code. It is possible that during PR reviews this changes and then this section should be updated.

How to test this PR locally

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

[elasticmachine]

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• Start Time: 2023-07-27T14:18:29.092+0000

• Duration: 36 min 37 sec

Test stats 🧪

Test	Results
Failed	0
Passed	751
Skipped	1
Total	752

💚 Flaky test report

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[mergify]

This pull request is now in conflicts. Could you fix it @mrodm? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b fix_retry_command_release_manager upstream/fix_retry_command_release_manager
git merge upstream/main
git push upstream fix_retry_command_release_manager