Fix enviroments seeding on certain branch names

What does this MR do?

Prevent seeding from blowing up on branch names that don't contain an alphanumerical character.

Are there points in the code the reviewer needs to double check?

The combination of first(10) and sample(4) was done to prevent people that seeded a large number of MRs to have to wait a long time. Also, I'm not quite sure about the style

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#45051