butane: replace LAYOUT with architecture before render

Fix failed test on aarch64 / ppc64le, the `.Subst("{{LAYOUT}}")`
is required for `raid1-boot` test, see coreos/fedora-coreos-config#4097

Author: HuijingHei

mantle/kola/harness.go

@@ -1374,7 +1374,9 @@ func registerTestDir(dir, testprefix string, children []os.DirEntry) error {
 			if err != nil {
 				return errors.Wrapf(err, "reading %s", c.Name())
 			}
-			userdata = conf.Butane(string(v))
+			// The .Subst("{{LAYOUT}}") is required for raid1-boot test, see
+			// https://github.com/coreos/fedora-coreos-config/pull/4097
+			userdata = conf.Butane(string(v)).Subst("{{LAYOUT}}", Options.CosaBuildArch)
 		} else if isreg && c.Name() == "kola.json" {
 			f, err := os.Open(fpath)
 			if err != nil {