fix cache signing config

[crazy-max]

relates to https://github.com/docker/buildx/actions/runs/20885866349/job/60008960190#step:10:541

#29 exporting to GitHub Actions Cache
#29 preparing build cache for export
#29 writing layer sha256:20ae2f90a4a205b860bfa6ff116489885225cbef9e2cbc8f93fd957f39321643
#29 writing layer sha256:20ae2f90a4a205b860bfa6ff116489885225cbef9e2cbc8f93fd957f39321643 0.5s done
#29 writing layer sha256:a785c78e2dcb466bd6296d5eb205c9d864a9589d8d645b7c950879ae5479e41f
#29 writing layer sha256:a785c78e2dcb466bd6296d5eb205c9d864a9589d8d645b7c950879ae5479e41f 2.5s done
#29 signing cache index sha256:f10fb1cd86490268d39dc7a75578719f31183203c71f00afbfaec176374bf36d
#29 preparing build cache for export 7.5s done
#29 signing cache index sha256:f10fb1cd86490268d39dc7a75578719f31183203c71f00afbfaec176374bf36d 1.7s done
#29 ERROR: signature verification failed: certificate field "githubWorkflowRepository" does not match policy ("docker/buildx" != "docker/github-builder-experimental")