fix: sort available cluster names

[MatousJobanek]

sort available cluster names to avoid flaky unit tests https://github.com/kubesaw/ksctl/actions/runs/10489485292/job/29054222888

[alexeykazakov]

Good catch!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.44%. Comparing base (3b26733) to head (def7c06). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #64 +/- ## ========================================== + Coverage 68.43% 68.44% +0.01% ========================================== Files 43 43 Lines 2474 2475 +1 ========================================== + Hits 1693 1694 +1 Misses 591 591 Partials 190 190 ``` | [Files](https://app.codecov.io/gh/kubesaw/ksctl/pull/64?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubesaw) | Coverage Δ | | |---|---|---| | [pkg/configuration/configuration.go](https://app.codecov.io/gh/kubesaw/ksctl/pull/64?src=pr&el=tree&filepath=pkg%2Fconfiguration%2Fconfiguration.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubesaw#diff-cGtnL2NvbmZpZ3VyYXRpb24vY29uZmlndXJhdGlvbi5nbw==) | `65.34% <100.00%> (+0.34%)` | :arrow_up: |