check for each plugin before running asdf install

What does this MR do?

./scripts/setup.sh fails if any of the asdf plugins in .tool-versions is not installed.

This MR adds a check before each call to asdf install that exits if a plugin is missing.

What are the relevant issue numbers?

Update setup.sh to handle missing asdf plugins (gitlab-org/gitlab#481836 - closed) • Jason Leasure • 17.5

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
  • Job definition example
  • CI Templates
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer