tmux module: Fix escaping in TMUX_TMPDIR

5404595b55 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.

nixos/modules/programs/tmux.nix

@@ -177,7 +177,7 @@ in {
       systemPackages = [ pkgs.tmux ];
 
       variables = {
-        TMUX_TMPDIR = lib.optional cfg.secureSocket ''''${XDG_RUNTIME_DIR:-"/run/user/\$(id -u)"}'';
+        TMUX_TMPDIR = lib.optional cfg.secureSocket ''''${XDG_RUNTIME_DIR:-"/run/user/$(id -u)"}'';
       };
     };
   };