Managing .openvex.json

.openvex.json carries per-CVE reachability evidence used to suppress vulnerability findings in the aiperf-bench container image. The file is consumed by the Weekly Image Vulnerability Scan workflow (.github/workflows/vuln-scan-images.yaml) via the vex: input on anchore/scan-action@v7.4.0, which passes it to grype as --vex .openvex.json.