libffi: only run tests on musl targets

Python3 is now needed for for glibc build which pulls libffi,
so I suppose libffi is now part of stdenv on linux with glibc
as on darwin.

pkgs/development/libraries/libffi/default.nix

@@ -1,9 +1,9 @@
 { stdenv, fetchurl, fetchpatch
 , autoreconfHook
 
-# libffi is used in darwin stdenv
+# libffi is used in darwin and linux with glibc stdenv
 # we cannot run checks within it
-, doCheck ? !stdenv.isDarwin, dejagnu
+, doCheck ? stdenv.hostPlatform.isMusl, dejagnu
 }:
 
 stdenv.mkDerivation rec {