internal/rosa: expose standalone musl

This is useful in the system image and might also be used elsewhere.

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

internal/rosa/musl.go

@@ -46,3 +46,8 @@ rmdir -v /work/lib
 		t.Load(Coreutils),
 	)...)
 }
+func init() {
+	artifactsF[musl] = func(t Toolchain) pkg.Artifact {
+		return t.newMusl(false, nil)
+	}
+}