Set sticky bit on WSLg X11 runtime directory

[Copilot]

WSLg creates the X11 socket directory used by XWayland, but the directory lacks the sticky bit, causing XWayland to refuse to start in GNOME/KDE scenarios. The fix must avoid the prior approach that removed the runtime directory creation.

• Behavioral fix
  • Keep creating /mnt/wslg/.X11-unix, but set its permissions to 01777 so the bind-mounted /tmp/.X11-unix satisfies XWayland’s sticky-bit requirement.

Example adjustment:

constexpr auto c_x11RuntimeDirMode = S_ISVTX | S_IRWXU | S_IRWXG | S_IRWXO;
THROW_LAST_ERROR_IF(chmod(c_x11RuntimeDir, c_x11RuntimeDirMode) < 0);

Original prompt

Please fix microsoft#1122 . Also, you cannot use the exact same operation as microsoft#1137 because it involves the restrictions of microsoft#1137 (comment) .

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