nixos/xserver: Don't use exec in xkb-layouts-exist

First of all, thanks to @pbogdan for getting this problem reproduced:

https://github.com/NixOS/nixpkgs/commit/2014db3efcd2a#commitcomment-22815396

Also thanks to @vcunat for bringing this to my attention:

https://github.com/NixOS/nixpkgs/commit/44c64fef16ed5#commitcomment-22813503



Although it is not entirely clear why Nix has killed the build prior to
finishing, it seems to be related to the process substition I was using.

So instead of using "exec touch", let's wrap this inside an if so we
don't exit too early.

Tested this against all sub-tests in nixos/tests/keymap.nix and also a
few configurations with wrong keyboard layout definitions.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>