refactor: Construct roles vector via iterator

[sweb]

Description

Minor refactoring I noticed while looking for usage examples of operator-rs

Definition of Done Checklist

I'd guess the author tasks do not apply here, but as a first time contributor, happy to get feedback on this first. I did add a unit test to make sure there is no regression in the function output - I removed it as I did not feel that this test provides a lot of long term value.

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[stackable-cla]

All committers have signed the CLA.

[Techassi]

Nice catch!