🌱 Minor improvements to api validation tests

[mdbooth]

This comprises 3 small cleanups:

• A makefile tidyup to how the KUBEBUILDER_ASSETS variable is set, which also allows it to be overridden
• Update the apivalidations to delete the objects it creates. Not really important, but might throw something up.
• Fix initialisation of the apivalidations envtest environment so that conversion webhooks work. Doesn't actually add any tests that use it, though.

/hold

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lentzi90, mdbooth

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/main/OWNERS)~~ [lentzi90,mdbooth] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[dulek]

/lgtm

[mdbooth]

/hold cancel