fix: Use fixed envtest version for integration tests

[c-pius]

Description

Changes proposed in this pull request:

• integration tests stopped working, see e.g., https://github.com/kyma-project/lifecycle-manager/actions/runs/13699632137/job/38315898587?pr=2295
• reason seems to be that we run integration tests using envtest@latest
  • the latest version has been updated to go 1.24 while we are still on 1.24
  • to fix that, proposing to use the previous version of envtest
  • I anyway think it is reasonable to rely on fixed versions
• the proposed ticket below is still valid, and of course we also need to migrate to go 1.24. So far we didn't because of the linting issue.

Related issue(s)

• The make test which run unit and integration tests is failing due to GOSUMDB #2296

[github-actions]

⚠️ Pipeline-related file changes detected! Please review if related updates (e.g. manifest generation or workflow adjustments) are required.