chore: Bring back kubectl to test-tools-image

[jannfis]

Brings back kubectl to test-tools-image, which was accidentally removed in #5101

Signed-off-by: jannfis jann@mistrust.net

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Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
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• My build is green (troubleshooting builds).

[alexmt]

LGTM