Prevent concurrent execution of PipelineScheduleWorker

What does this MR do?

Looking at https://log.gitlab.net/goto/ac839ad8c9bd011133637fd62251a6ea, PipelineScheduleWorker is irregularly executed regardless of its "19 * * * *" interval. We should prevent this worker from concurrent execution otherwise a new pipeline will be created in a short interval and it weighs in production load (and it could be a root cause of the weird behavior).

This patch should go to either post-production patch or the next patch release.

Close gitlab-com/gl-infra/production#805 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
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Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team