Batch-load environments in EE pipelines

What does this MR do?

This fixes an N+1 in PipelineSerializer (and possibly elsewhere) for EE clients that use protected environments.

Previously, this would re-load the environment for every build being rendered. We realized that instead we can just make a call to Build#expanded_environment_name, which does not incur a round-trip to the database.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #321788