internal/rosa: fix toolchain layer order

This allows extras to override toolchain artifacts.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/rosa.go

@@ -215,14 +215,14 @@ ln -vs ../usr/bin /work/bin
 	case toolchainIntermediate, Std:
 		boot := t - 1
 		musl, compilerRT, runtimes, clang := boot.NewLLVM()
-		support = slices.Concat([]pkg.Artifact{
+		support = slices.Concat(extra, []pkg.Artifact{
 			cureEtc{newIANAEtc()},
 			musl,
 			compilerRT,
 			runtimes,
 			clang,
 			boot.NewBusybox(),
-		}, extra)
+		})
 		env = fixupEnviron(env, []string{
 			EnvTriplet + "=" + triplet(),
 			lcMessages,