Use 'caproni kubectl' in exec script

What does this MR do and why?

Use 'caproni kubectl' in exec script

This replaces the use of the bare kubectl call with caproni kubectl in the .gitlab/caproni/exec.sh script. This works for running in lifecycle hooks because we pass the KUBECONFIG environment variable to the script. But the gitlab:rspec task in gitlab-caproni as well as calling it directly on the CLI causes kubectl to fall back to the global kube config, which Caproni no longer automatically writes to.

This ensures that we talk to the right cluster based on the top-level caproni.yaml file.

References

Closes #628149 (closed)

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