ci: use environment variables inheritance

What does this MR do?

This changes the CI config to make use of environment variables inheritance to pass the $DEPENDENCY_URL variable to the create_integration_branch job.

This requires the ci_dependency_variables feature flag to be enabled.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.

Closes #574 (closed)