internal/rosa: raise stage0 extra layers

This enables extras to override stage0 tarball.

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

internal/rosa/rosa.go

@@ -298,6 +298,7 @@ func (t Toolchain) New(
 
 	case toolchainGentoo, toolchainStage0:
 		name += "-boot"
+		support = append(support, extra...)
 		support = append(support, cureEtc{})
 		if t == toolchainStage0 {
 			support = append(support, NewStage0())
@@ -318,7 +319,6 @@ mkdir -vp /work/system/bin
 				),
 			)))
 		}
-		support = slices.Concat(support, extra)
 		env = fixupEnviron(env, []string{
 			EnvTriplet + "=" + triplet(),
 			lcMessages,