_v2-entry-point: Use a shorter rendezvous directory name
The AF_UNIX sockets that are used as rendezvous-point are limited to 108 bytes by the Linux kernel.
To prevent exceeding this limit, especially when we use the XDG_RUNTIME_DIR prefix, we use a shorter directory name that is still distinctive enough.
Related to steam-runtime-tools!264 (closed), but it's not a hard requirement.