Returns a directory that is unique to the current user on the local system.
This is determined using the mechanisms described
XDG Base Directory Specification.
This is the directory
specified in the
XDG_RUNTIME_DIR environment variable.
In the case that this variable is not set, we return the value of
g_get_user_cache_dir(), after verifying that it exists.
The return value is cached and modifying it at runtime is not supported, as it’s not thread-safe to modify environment variables at runtime.