Avoid repeated dashes in $CI_ENVIRONMENT_SLUG

What does this MR do?

Avoids repeated - characters in $CI_ENVIRONMENT_SLUG and fixes an edge-case where 'env-' wouldn't be prepended to environments beginning with '-'.

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

Should we rename existing slugs? I don't believe we should, as this has a chance of breaking existing, running environments.

Why was this MR needed?

Turns out a name like foo--bar is invalid in OpenShift.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26852 (closed)