Replace deprecated xorg.libXtst with libxtst

[archandanime]

Replace pkgs.xorg.libXtst with pkgs.libxtst in flake.nix

This resolves the NixOS 26.05 evaluation warning: evaluation warning: The xorg package set has been deprecated, 'xorg.libXtst' has been renamed to 'libxtst'

Tested and verified building successfully on NixOS 26.05:

$ sudo nixos-rebuild switch --flake path:/etc/nixos
warning: updating lock file "/etc/nixos/flake.lock":
• Updated input 'ibus-bamboo':
    'github:BambooEngine/ibus-bamboo/49e03d7a36b8a8b796950c158b545af8cf9707d4?narHash=sha256-1qj1Fx8RDdA1qEGWn%2B/nQCAu4HCei%2BYbF7SHDzuFw2I%3D' (2026-05-19)
  → 'github:archandanime/ibus-bamboo/3e07c6b16a244a23c06701722574bffc4ff7b575?narHash=sha256-cSRXG4UaCScvHsy8DpCjcosE%2BbhG4aNnfxM34SLHJzI%3D' (2026-08-07)
building the system configuration...
Checking switch inhibitors... done
Installing Lanzaboote to "/boot"...
Collecting garbage...
Successfully installed Lanzaboote.
stopping the following units: accounts-daemon.service
activating the configuration...
setting up /etc...
Initializing permissions on persistent paths...
reloading user units for user...
stopping the following user units: org.freedesktop.IBus.session.GNOME.service
reloading the following user units: dbus-broker.service
starting the following user units: org.freedesktop.IBus.session.GNOME.service
restarting the following user units: nixos-activation.service
restarting sysinit-reactivation.target
reloading the following units: dbus-broker.service
restarting the following units: polkit.service
starting the following units: accounts-daemon.service
the following new units were started: NetworkManager-dispatcher.service
Done. The new configuration is /nix/store/pbqk26sy3d7p0b6sw5cfcsaiqg11n7sz-nixos-system-nixos-26.05.20260731.5b4f72e