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"},

mantle/kola/tests/ignition/qemufailure.go

@@ -227,7 +227,6 @@ func dualBootfsFailure(c cluster.TestCluster) error {
 	}
 	builder.MemoryMiB = 1024
 	builder.Firmware = kola.QEMUOptions.Firmware
-
 	searchRegexString := "Error: System has 2 devices with a filesystem labeled 'boot'"
 	if err := verifyError(builder, searchRegexString); err != nil {
 		return err