ci: publish openvex on release

[ricardoapl]

What this PR does / why we need it:

Append OpenVEX data to release, not just to stdout

How does this change affect the cardinality of KSM:

Does not change cardinality

Which issue(s) this PR fixes:

Part of #2274

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[mrueg]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mrueg, ricardoapl

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