gnutls: set certificate location (close #8121)

This is a fast fix; it might be best to use $SSL_CERT_FILE.
Tested on vlc with youtube https URLs.
Discussed also on #8118. Feel free to discuss further improvements on #8247.

pkgs/development/libraries/gnutls/generic.nix

···
   inherit src patches;
 
   configureFlags = [
+    # FIXME: perhaps use $SSL_CERT_FILE instead
+    "--with-default-trust-store-file=/etc/ssl/certs/ca-certificates.crt"
     "--disable-dependency-tracking"
     "--enable-fast-install"
   ] ++ stdenv.lib.optional guileBindings