Use 'assemble' task for gradle projects

What does this MR do?

In MR !130 (merged), the gradlew builder was modified to run the assemble gradle task instead of build, allowing to skip unit tests. Unfortunately, the gradle builder was forgotten and is still running the build task. This MR fixes that.

What are the relevant issue numbers?

None

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer