CI: ubuntu-musl target

vlib/builtin/builtin_nix.v

@@ -24,9 +24,11 @@ fn print_backtrace_skipping_top_frames_nix(xskipframes int) bool {
 // the functions below are not called outside this file,
 // so there is no need to have their twins in builtin_windows.v
 fn print_backtrace_skipping_top_frames_mac(skipframes int) bool {
+	$if mac {
 	buffer := [100]byteptr
 	nr_ptrs := C.backtrace(*voidptr(buffer), 100)
 	C.backtrace_symbols_fd(*voidptr(&buffer[skipframes]), nr_ptrs-skipframes, 1)
+	}
 	return true
 }