5404595b55eeefae69f2952a34dfc70e32523060 relocated code but kept
one backslah too many, leading to
$ tmux
error creating /run/user/$(id -u)/tmux-1000 (No such file or directory)
/run/user/$UID/ is created by pam_systemd(3) which also populates
XD_RUNTIME_DIR with that value.
Alternatively, TMUX_TMPDIR might simply default to XDG_RUNTIME_DIR
without providing the same directory yet again as default string in
parameter substitution, however such behaviour change is subject to
another patch.
In fact, with `security.polkit.enable = false` systemd_logind(8) fails
to start and /run/user/$UID/ is never created for unprivileged users
in proper login sessions; XDG_RUNTIME_DIR would consequently not be
set either.
Removing the fallback to /run/user/$UID/ would have caused TMUX_TMPDIR
to be empty, which in turn would lead tmux(1) to use /tmp/. This
effectively breaks the idea of isolated sockets entirely while hiding
errors from the user.
7f104aa6e2