kola: run luks.cex test only on RHCOS.s390x

Test requires some magic https://github.com/coreos/fedora-coreos-pipeline/blob/main/multi-arch-builders/coreos-s390x-rhcos-builder.bu#L107,
which only exists on RHCOS builders.

Author: nikita-dubrovskii

mantle/kola/tests/ignition/luks.go

@@ -58,6 +58,7 @@ func init() {
 		Name:          `luks.cex`,
 		Description:   "Verify that CEX-based rootfs encryption works.",
 		Flags:         []register.Flag{},
+		Distros:       []string{"rhcos"},
 		Platforms:     []string{"qemu"},
 		Architectures: []string{"s390x"},
 		Tags:          []string{"luks", "cex", "reprovision"},