internal/rosa: replace busybox with toybox

The busybox artifact does not run on aarch64, and the workarounds required for it to compile successfully on x86_64 is unacceptably shaky. This change fully replaces it with toybox.

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

internal/rosa/musl.go

@@ -44,6 +44,7 @@ rmdir -v /work/lib
 
 	return t.New("musl-"+version, false, stage3Concat(t, attr.Extra,
 		t.Load(Make),
+		t.Load(Coreutils),
 	), nil, slices.Concat([]string{
 		"ROSA_MUSL_TARGET=" + target,
 	}, attr.Env), `