fix(vex): handle 304 status code

[alegrey91]

Description

Currently, trivy deletes the content of the vex cache every time a new download happens.
This action is not always needed, since when an ETag is provided, the server may respond with 304 Not Modified, meaning the existing content at dst is still valid.
In that case, trivy should not destroy dst. Instead, we can move it aside as a backup and restore it on 304.
This particular case happens when a Vexhub repository has a small ' update_interval '.
Here's the original PR where we noticed the bug: kubewarden/sbomscanner#867
cc @fabriziosestito

Related issues

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