BuildKit: Custom frontend could bypass Seccomp/AppArmor

BuildKit is a toolkit for converting source code to build artifacts in an efficient, expressive and repeatable manner. Prior to 0.31.1, a custom frontend could place an invalid SecurityMode value in a crafted build request, and executor/oci/spec_linux.go treated the unsupported value as a non-sandbox mode without requiring the security.insecure entitlement. This disabled Seccomp and AppArmor protections for the build container even though Linux capabilities remained restricted. This issue is fixed in version 0.31.1.