Fix duplicate environment name causes pipeline creation failure

What does this MR do?

The MR, which was recently shipped in 12.4, contained a bug as reported in #34444 (closed). When there are duplicate environment:name in the same pipeline (.gitlab-ci.yml), the pipeline fails to be persisted because of the unique constraint on the environment.name.

For example, in the following .gitlab-ci.yml, pipeline cannot be created.

deploy:
  environment: production

deploy-2:
  environment: production

Because, there are two jobs point to the same environment production. CreatePipelineService tries to save multiple Environment.new(name: 'production', project: project) and violates uniqueness: { scope: :project_id } validation. The unpersisted environment object is assigned to a deployment object and results in NotNullViolation on environment_id.

On the other hand, typical review app workflows are unaffected, as stop-review doesn't create environment object as action: stop.

review:
  environment:
    name: review/$CI_COMMIT_REF_NAME
    on_stop: stop-review

stop-review:
  environment:
    name: review/$CI_COMMIT_REF_NAME
    action: stop

This behavior was reproduced on staging. https://staging.gitlab.com/dosuken123/deployment-iid-test/merge_requests/3/diffs => https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/1022739/?query=is:unresolved.

Close #34444 (closed)

Does this MR meet the acceptance criteria?

Conformity

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

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