proxsuite: fix build with clang (#368311)

  # Fontconfig error: Cannot load default config file: No such file: (null)

  env.FONTCONFIG_FILE = "${fontconfig.out}/etc/fonts/fonts.conf";

  env.NIX_CFLAGS_COMPILE = toString (

    lib.optionals stdenv.cc.isClang [

      "-Wno-error=missing-template-arg-list-after-template-kw"

    ]

  );

  # Fontconfig error: No writable cache directories

  preBuild = "export XDG_CACHE_HOME=$(mktemp -d)";